Extend the syntax of #line and __FILE__ to support filename strings.

The implementation is done via introducing a new extension 
GL_GOOGLE_cpp_style_line_directive using the extension framework.

The purpose is to support cpp-style #line directives, which is
required by #include.

Use requireExtensions() to check GL_GOOGLE_cpp_style_line_directive
is turned on. Add register more tests for running.

According to the GLSL spec, the second parameter to #line should be
an integer source string number and __FILE__ will be substituted
with the integer source string number currently processed. This
patch extends the syntax of #line and __FILE__. Now #line accepts
as the second parameter a filename string quoted by double quotation
marks. And if such a #line is set, __FILE__ will be substituted with
the currently set filename string. The implementation is done via
introducing a new extension GL_GOOGLE_cpp_style_line_directive using
the extension framework.

The purpose is to support cpp-style #line directives, which is
required by #include.

Scanning: 1) rationalize end-of-input everywhere, 2) prevent infinite loop at end-of-input, 3) use positive chars.

Fixes issue #25. (char 255 aliased to -1 and missing tests for end of input).

1) All layers of input scanning now share a single EndOfInput value.
This avoids translation of it across layers of encapsulation.

2) Some places looking for end of line were not stopping on EndOfInput.

3) Use of "char" for the input made char values > 127 be negative numbers.
This allowed for aliasing of 255 to -1, etc.  This is fixed by using
unsigned char.

This is just for '\' that's not before a new line.
Note the specification says it has no use other than as line continuation,
but #error is a grey area. (There are no escape sequences.)

Consolidate all token length limits (1024), including one bad one (80) hardcoded on the stack (issue #40, issue #41).

Fix missing const qualifier on TUnorderedMap template type

Preprocessing line number handling

Everytime we switch to a new source string, the line are reset.
We need to reset lastLine and also output newlines appropriately.

SourceLineSynchronizer is added for keeping track of the last
line number and output newlines appropriately if we switch to
a new line in the preprocessor.

AST -> SPV testing: Update test for using new TbdFunctionality() that is non-fatal and uniquifies printed TBDs.

Only output explicit extension directives in preprocessing.

spv::Builder::Loop constructor inits all members.

After construction, the Loop is effectively const.

This perturbs the IDs in SPIR-V tests because the body block
is created before generating any of the loop code, rather than
only when the body is first referenced.

Issue #28: Correctly handle a single input argument conversion when the input argument is an aggregate.

Preprocessing output shouldn't contain extensions enabled as
dependencies.

Generate correctly structured do-while loops.

Fix g++ build break (portibility problems with stl hash).  This partly turns off the stl improvements.

It also removes some old code that ancient compilers used to need.
However, the main issue is getting access to hash functions for
unordered_map in portable way.

Preprocessor: 1) Rationalize the "fixed atom" scheme, 2) remove redundant lookups when the text is already available.

This simplification is a prelude to eliminating what I appear unnecessary
symbol inserts into tables when tokenizing in the preprecessor, which
show up as taking notable time. (Performance issue.)  It also simply makes
the preprocessor easier to understand, which it is badly in need of.

Front-end: Add error-recovery code for a switch statement that ends with a case/default that has no statements.

The loop test is always emitted before the loop body.

For do-while loops, use a phi node to track whether we're
on the first loop iteration, and only check the loop test
on the second and subsequent iterations.

For do-while loops, the loop test branch no longer occurs
at the top of the loop, so it must get its own selection
merge instruction.

A block can't be the target of more than one merge instruction.
So when the loop test executes after the body (as in do-while in GLSL)
we need to introduce a dummy block to be the target of the selection
merge just before the loop test conditional branch.

The other arm of the branch exits the loop and hence is the
"break block" exception in the structured control flow rules.