[HLSL] Add support for `SV_Barycentrics`

Add `SV_Barycentrics` to the list of parsed semantics and lower as follows:

DXIL: `LoadInput` op (`dx.op.loadInput`, opcode 4)
SPIRV: `BaryCoordKHR` built-in load (`BaryCoordNoPerspKHR` when declared `noperspective`).

Reference [here](https://github.com/llvm/wg-hlsl/blob/main/proposals/0040-semantics-overview.md#system-value-constraints) for its type constraints. Reference the [pixel](https://github.com/llvm/wg-hlsl/blob/main/proposals/0040-semantics-overview.md#pixel-shader) shader sections for packing info.

AC:
- [ ] Add the semantic spelling in `Attr.td`
- [ ] Update `CGHLSLRuntime` to handle the semantic spelling
- [ ] Update `SemaHLSL` to add additional diagnostics
- [ ] Update shader flag analysis to set the `Barycentrics` flag when this semantic is present
- [ ] Add LIT testing of sema and codegen