The `dx.dot2`, `dot3`, and `dot4` intrinsics exist purely to lower
`dx.fdot`, and they map exactly to the DXIL ops of the same name. Using
vectors for their arguments adds unnecessary complexity and causes us to
have vector operations that are not trivial to lower post-scalarizer.

Similarly, the `dx.dot2add` intrinsic is overly generic for something
that only needs to lower to a single `dot2AddHalf` DXIL op. Update its
signature to match the operation it lowers to.

Fixes #134569.

- fixes #132303
- Moves dot2add from a language builtin to a target builtin.
-  Sets the scaffolding for Sema checks for DX builtins
-  Setup DirectX backend as able to have target builtins
- Adds a DX TargetBuiltins emitter in
`clang/lib/CodeGen/TargetBuiltins/DirectX.cpp`