[HLSL] Implement `HLSLGloballyCoherentAttr`

Implement the `globallycoherent` qualifier in the Clang front end: parse it, represent it on the resource type in the AST, and add the Sema checks that constrain where it may appear.

_Note_: This does not include the codegen component that is yet to be designed

AC:

- [ ] Add the `HLSLGloballyCoherent` attribute to `Attr.td` with the `globallycoherent` spelling
- [ ] Parse the qualifier and attach it to the resource declaration / type in the AST
- [ ] Sema: Diagnose the qualifier on non-UAV objects ("'globallycoherent' can
only be applied to UAV objects")
- [ ] Sema: Handle add/drop of the qualifier across pointer/reference conversions
- [ ] Add LIT tests of the ast-dump
- [ ] Add Sema tests of diagnostics