[HLSL] Added HLSL this as a reference

This change makes `this` a reference instead of a pointer in
HLSL. HLSL does not have the `->` operator, and accesses through `this`
are with the `.` syntax.

Tests were added and altered to make sure
the AST accurately reflects the types.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D135721