Array constructors are not folded, unlike all other constant expressions.
Change initializer parsing path so that it accepts constant initializers
whether they are folded or not.

Some parts need to be adapted to work with expressions that are qualified
as constant but that are not necessarily folded:

1. Identifier parsing
2. Indexing parsing
3. Field selection parsing
4. HLSL output for variable declarations
5. Determining unary operator result type
6. Determining binary operator result type
7. Determining built-in function call result type
8. Determining ternary operator result type

Corner cases that are not supported yet:

1. Using array constructors inside case labels
2. Using array constructors inside array size expressions
3. Detecting when a negative constant expression containing an array
   constructor is used to index an array

In these cases being able to constant fold the expression is essential to
validating that the code is correct, so they require a more sophisticated
solution. For now we keep the old code that rejects the shader if ANGLE
hasn't been able to constant fold the case label or array size. In case of
indexing an array with a negative constant expression containing an array
constructor, ANGLE will simply treat it as a non-constant expression.

BUG=541551
BUG=angleproject:1094
TEST=dEQP-GLES3.functional.shaders.constant_expressions.* (all pass),
     angle_unittests

Change-Id: I0cbc47afd1651a4dece3d68acf7ec72a01fdf047
Reviewed-on: https://chromium-review.googlesource.com/310231Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>