[HLSL] Implement availability diagnostic - Default and Relaxed modes

[hekota]

Implement default and relaxed modes of the availability diagnostic.

The default diagnostic mode performs an AST traversal after the translation unit has been fully parsed, and requires construction of a call graph. An AST visitor will traverse to all CallExpr nodes that are reachable from exported functions (either library exports or entry functions). If the callee of a CallExpr has availability annotations that signify that the API is unavailable for the target shader model and stage the compiler emits an error.

The implementation of the relaxed diagnostic mode matches the default mode, except that when a CallExpr references an unavailable API, the compiler emits a warning. A user enables relaxed mode by passing -Wno-error=hlsl-availability.

See HLSL Availability Diagnostic spec for more details.