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.

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().

SPV test: Test dead-decoration removal.

SPV: Remove decoration of undefined IDs

Preprocessor: Fix array out of bounds bug in processing if-else-endif macros.

Fix issue #185 by removing OpDecorate instructions whose target IDs are
defined in unreachable blocks and thus not dumped in the generated
SPIR-V code.

If we are not inside an if macro, we cannot simply decrease
elsetracker.

Fixes https://github.com/KhronosGroup/glslang/issues/29.

SPV: Declare MultiViewport capability.

Fix issue #191:
https://github.com/KhronosGroup/glslang/issues/191

SPV: Add support for memory qualifiers.

SPV: Support the new OpCode - OpImageSparseRead

Remove bison binary

SPV: Continue to fix the issue of bool -> uint32

Fix line endings in Test/100.conf

For short-circuit operator (&& and ||), the conversion is missing.