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.

Parser: Add 64-bit type conversion for specialization constant.