[CMake] Add `CLANG_ENABLE_HLSL` CMake option

The HLSL support in clang is in progress and not fully functioning. As
such we don't want to install the related optional build components by
default (yet), but we do need an option to build and install them
locally for testing and for some key users.

This adds the `CLANG_ENABLE_HLSL` option which is off by default and can
be enabled to install the HLSL clang headers and the clang-dxc symlink.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D134693