This adds support for #pragma pack_matrix() to the HLSL front end.

The pragma sets the default matrix layout for subsequent unqualified matrices
in structs or buffers. Explicit qualification overrides the pragma value. Matrix
layout is not permitted at the structure level in HLSL, so only leaves which are
matrix types can be so qualified.

Note that due to the semantic (not layout) difference in first matrix indirections
between HLSL and SPIR-V, the sense of row and column major are flipped.  That's
independent of this PR: just a factor to note.  A column_major qualifier appears
as a RowMajor member decoration in SPIR-V modules, and vice versa.