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.

HLSL: Add location offsets per resource type

This PR adds the ability to offset sampler, texture, and UBO bindings
from provided base bindings, and to auto-number bindings that are not
provided with explicit register numbers. The mechanism works as
follows:

- Offsets may be given on the command line for all stages, or
  individually for one or more single stages, in which case the
  offset will be auto-selected according to the stage being
  compiled. There is also an API to set them. The new command line
  options are --shift-sampler-binding, --shift-texture-binding, and
  --shift-UBO-binding.

- Uniforms which are not given explicit bindings in the source code
  are auto-numbered if and only if they are in live code as
  determined by the algorithm used to build the reflection
  database, and the --auto-map-bindings option is given. This auto-numbering
  avoids using any binding slots which were explicitly provided in
  the code, whether or not that explicit use was live. E.g, "uniform
  Texture1D foo : register(t3);" with --shift-texture-binding 10 will
  reserve binding 13, whether or not foo is used in live code.

- Shorter synonyms for the command line options are available.  See
  the --help output.

The testing infrastructure is slightly extended to allow use of the
binding offset API, and two new tests spv.register.(no)autoassign.frag are
added for comparing the resulting SPIR-V.

This is not a claim that reflection is working right, only a way to
see it is occurring and catch any regression.

This includes the "< decl ; decl ; >" syntax which has its own namespace.
This functionality is not implemented, just silently accepted.

... and use each in the correct way at consumption sites.
This completes issue #513.

Refactor TLiveTraverser from the former reflection traverser, for fut…

This PR factors out the code that knows how to walk just the live parts of the AST.
The traverser in reflect.cpp is renamed to TReflectionTraverser, and inherits from
TLiveTraverser, which will also be used by a future binding offset PR.

The code is now smart about the entry point name (no longer hardcoded to "main").

There is an option to traverse all code (live+dead), because a consumer of the
class may wish to use it for both purposes without wanting a whole separate
class hierarchy.

Partially addresses issue #513.

Fixes issue #510.

Notably, use of 'linear' on a non-input could mark it as an input.

This is needed because an output structure can contain embedded built-ins
(like SV_Position) which should not get locations assigned.

Locations now get assigned in order, but skipping built-ins, which can be
done post flattening.

At least partially addresses issue #510.

SPV: Support simultaneous l-value swizzle and dynamic component selection.

This makes the interface be invariant, whether or not individual
variables are used.

Vulkan can't handle structures into the vertex stage or out
of the fragment stage.

HLSL: alias HLSL matrix-row-column onto AST matrix-column-row

WIP: HLSL: EOpGenMul arg reversal

SPV: Use OpBitcast to implement 4 pack/unpack built-in functions.

This was suggested in issue #454.

This will prevent a possible future defect of thinking the type can be changed,
where there is a code path today that would drop that change.