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.

Addresses issue #304 and issue #307 by replacing unmatched type OpStores with
per-member copies. Covers assignment statements and most argument passing, but
does not yet cover r-value-based argument passing.

Remapper: move remapper tests into the glslangtests executable.