Move SpecConstantOpModeGuard from makeSpvConstantFromConstSubTree() to
visitbinary() and visitunary(). Checking if the visiting node is a spec
constants, if so, turn on the SpecConstantOpMode, otherwise, stay in the
normal mode.

fix the wrong generated code when a non-constant array is declared with its size derived from spec constant operations

Handle built-in constants redeclared with a specialization constant id.

Support declaring built-in values as spec constants.

Refine the code in createSpvConstant().

VS2010 compile fixes

Fix spelling mistakes

Add Google Test as the new test framework

Parser: Update array size of gl_ClipDistance/gl_CullDistance in gl_in[].

SPV: Spec Constant Operations

fix format; remove unnecessary parameters; rename some spec op mode guard class; remove support of float point comparison and composite type comparison; update the tests.

The existing test harness is a homemade shell script. All the tests
and the expected results are written in plain text files. The harness
just reads in a test, invoke the glslangValidator binary on it, and
compare the result with the golden file. All tests are kinda
integration tests.

This patch add Google Test as an external project, which provides a
new harness for reading shader source files, compile to SPIR-V, and
then compare with the expected output.

Approach:
Add a flag in `Builder` to indicate 'spec constant mode' and 'normal
mode'. When the builder is in 'normal mode', nothing changed. When the
builder is in 'spec constant mode', binary, unary and other instruction
creation rountines will be redirected to `createSpecConstantOp()` to
create instrution at module level with `OpSpecConstantOp <original
opcode> <operands>`.

'spec constant mode' should be enabled if and only if we are creating
spec constants. So a flager setter/recover guard is added when handling
binary/unary nodes in `createSpvConstantsFromConstSubTree()`.

Note when handling spec constants which are represented as ConstantUnion
Node, we should not use `OpSpecConstantOp` to initialize the composite
constant, so builder is set to 'normal mode'.

Tests:
Tests are added in Test/spv.specConstantOperations.vert, including:

1) Arithmetic, shift opeations for both scalar and composite type spec constants.
2) Size conversion from/to float and double for both scalar and vector.
3) Bitwise and/or/xor for both scalar and vector.
4) Unary negate/not for both scalar and vector.
5) Vector swizzles.
6) Comparisons for scalars.
7) == and != for composite type spec constants

Issues:
1) To implement == and != for composite type spec constants, the Spec needs
to allow OpAll, OpAny, OpFOrdEqual, OpFUnordEqual, OpOrdNotEqual,
OpFUnordNotEqual. Currently none of them are allowed in the Spec.

SPV: Support specialization composite constants.

Fix issue #163, support creation and reference of composite type
specialization constants.

e.g.:
```
layout(constant_id = 200) const float myfloat = 1.25;
layout(constant_id = 201) const int myint = 14;
struct structtype {
  float f;
  int i;
};
const structtype outer_struct_var = {myfloat, myint};
void main(){}
```
generated code (use glslangValidator):
```
// Module Version 10000
// Generated by (magic number): 80001
// Id's are bound by 12

                              Capability Shader
               1:             ExtInstImport  "GLSL.std.450"
                              MemoryModel Logical GLSL450
                              EntryPoint Vertex 4  "main"
                              Source GLSL 450
                              Name 4  "main"
                              Name 10  "structtype"
                              MemberName 10(structtype) 0  "f"
                              MemberName 10(structtype) 1  "i"
                              Decorate 7 SpecId 200
                              Decorate 9 SpecId 201
               2:             TypeVoid
               3:             TypeFunction 2
               6:             TypeFloat 32
               7:    6(float) SpecConstant 1067450368
               8:             TypeInt 32 1
               9:      8(int) SpecConstant 14
  10(structtype):             TypeStruct 6(float) 8(int)
              11:10(structtype) SpecConstantComposite 7 9
         4(main):           2 Function None 3
               5:             Label
                              Return
                              FunctionEnd
```

Rname two function names to match their functionalities.
1) Rename `GlslangToSpvTraverser::createSpvSpecConstant()` to
`createSpvConstant()`;
2) Rename `GlslangToSpvTraverser::createSpvConstant()` to
`createSpvConstantFromConstUnionArray()`

Add function `GlslangToSpvTraverser::createSpvConstantFromSubTree()` to
handle constant creation from sub trees (e.g.: specialization constants).

Related PR: #208

VS2012: Fix a build issue in parser.

Fixed compilation issue introduced by my last commit

Updated the includer interface to allow relative includes.

This plumbs both the current file path and the include depth
back up to the includer. This allows the includer to properly
support relative paths.

This also replaces the string copy that was done during include
with a zero-copy method of accomplishing the same thing. This
prevents extra copies of entire files.

Much about const or temp is mechanical, about actual declaration,
while much is semantic, about something higher level.  This commit
checks every use everywhere, and for the high-level ones, substitutes
an encapsulated version instead.

Parser: Type promotion for operator modulus(%) is missing.

Support Linux-to-Windows cross compile via MinGW

Change-Id: Ie52f0b1b2b20948c6f4b3cb5474537d36a5a3385

SPV: Implement boolean selection for mix().

Use OpSelect to implement boolean selection of mix(). FMix is applicable
to linear blending of mix().