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.

SPV: OpGroupBroadcast is unable to handle vector operand.

GLSL: Add IO map tests

This PR adds a GLSL equivalent to the HLSL binding mapping tests for offsets and auto-numbering.
The shaders are as equivalent as possible.  The bindings of the base results match exactly
between the two.

SPV: Still have to specify SPIR-V extension for gl_SubGroupSizeARB and gl_SubGroupInvocationARB.

Fix defects in uniform array flattening

Fix for two defects as follows:

- The IO mapping traverser was not setting inVisit, and would skip some AST nodes.
  Depending on the order of nodes, this could have prevented the binding from
  showing up in the generated SPIR-V.

- If a uniform array was flattened, each of the flattened scalars from the array
  is still a (now-scalar) uniform.  It was being converted to a temporary.

SPV: Implement the extension SPV_KHR_shader_ballot

HLSL: Flatten uniform arrays

This checkin adds a --flatten-uniform-arrays option which can break
uniform arrays of samplers, textures, or UBOs up into individual
scalars named (e.g) myarray[0], myarray[1], etc.  These appear as
individual linkage objects.

Code notes:

- shouldFlatten internally calls shouldFlattenIO, and shouldFlattenUniform,
  but is the only flattening query directly called.

- flattenVariable will handle structs or arrays (but not yet arrayed structs;
  this is tested an an error is generated).

- There's some error checking around unhandled situations.  E.g, flattening
  uniform arrays with initializer lists is not implemented.

- This piggybacks on as much of the existing mechanism for struct flattening
  as it can.  E.g, it uses the same flattenMap, and the same
  flattenAccess() method.

- handleAssign() has been generalized to cope with either structs or arrays.

- Extended test infrastructure to test flattening ability.