HLSL: phase 1: add RWTexture and RWBuffer

There's a lot to do for RWTexture and RWBuffer, so it will be broken up into
several PRs.  This is #1.

This adds RWTexture and RWBuffer support, with the following limitations:
  * Only 4 component formats supported
  * No operator[] yet

Those will be added in other PRs.

This PR supports declarations and the Load & GetDimensions methods.  New tests are
added.

Gtests can be run on another source tree

The gtest executable accepts a --test-root option to specify
a root directory for test files.  It defaults to the Test directory
in the source tree from which the executable is built.

For example, this lets us run test exectuables built with MinGW on Linux
on a Windows machine with its own copy of the source tree.

HLSL: fix for flattening assignments from non-symbol R-values.

If a member-wise assignment from a non-flattened struct to a flattened struct sees a complex R-value
(not a symbol), it now creates a temporary to hold that value, to avoid repeating the R-value.
This avoids, e.g, duplicating a whole function call.  Also, it avoids re-using the AST node, making a
new one for each member inside the member loop.

The latter (re-use of AST node) was also an issue in the GetDimensions intrinsic decomposition,
so this PR fixes that one too.

add reflection queries to return a TType.  Fix minor issue with inter…

- Add new queries: TProgram::getUniformTType and getUniformBlockTType,
  which return a const TType*, or nullptr on a bad index.  These are valid for
  any source language.

- Interface name for HLSL cbuffers is taken from the (only) available declaration name,
  whereas before it was always an empty string, which caused some troubles with reflection
  mapping them all to the same index slot.  This also makes it appear in the SPIR-V binary
  instead of an empty string.

- Print the binding as part of the reflection textual dump.

- TType::clone becomes const.  Needed to call it from a const method, and anyway it doesn't
  change the object it's called on.

- Because the TObjectReflection constructor is called with a TType *reference* (not pointer)
  so that it's guaranteed to pass in a type, and the "badReflection" value should use a nullptr
  there, that now has a dedicated static method to obtain the bad value.  It uses a private
  constructor, so external users can't create one with a nullptr type.

Non-functional: Rationalizing parse helper hierarchy, step 3 (effected editable symbols and IO resize).

Non-functional: Rationalizing parse helper hierarchy, step 2 (effected error messaging and cascading errors).

Change binding auto-map to use provided offsets.

Previously, the binding auto-mapping facility was free to use any unused
binding.  This change makes auto-bindings use the same offset value as
explicit bindings.

HLSL: Restrict uniform array flattening to sampler and texture arrays

SPV: PrimitiveId in frag shader will emit OpCapability

HLSL: Add a test for buffer auto-binding assignment.

Parser: Implement extension GL_AMD_gpu_shader_half_float.

- Add built-in types: float16_t, f16vec, f16mat.
- Add support of half float constant: hf, HF.
- Extend built-in floating-point operators: +, -, *, /, ++, --, +=, -=,
  *=, /=, ==, !=, >=, <=, >, <.
- Add support of type conversions: float16_t -> XXX, XXX -> float16_t.
- Add new built-in functions.

Updated README.md

Added an alternative link for cloning the repo in the build instructions

Previously the uniform array flattening feature would trigger on loose
uniform arrays of any basic type (e.g, floats).  This PR restricts it
to sampler and texture arrays.  Other arrays would end up in their own
uniform block (anonymous or otherwise).  (Atomic counter arrays might be an
exception, but those are not currently flattened).

Used initially just by HLSL, for $Global.  Could be an option
for GLSL -> Vulkan.

HLSL: allow implicit size in array declarations

In HLSL array sizes need not be provided explicitly in all circumstances.
For example, this is valid (note no number between the [ ]):

  // no explicit array size
  uniform float g_array[] = { 1, 2, 3, 4, 5 };

This PR does not attempt to validate most invalid cases.

A new test is added to verify the resulting linker objects.

#517: Enable AMD extensions by default

Using PrimitiveId in a fragment shader requires declaring
an OpCapability with either Geometry or Tessellation.