[clang] [CMake][CodeGen] Add PCH for Clang CodeGen (PR #206018)

[Alexis Engelke via cfe-commits]

aengelke wrote:

This over-compensates the regression of #205986 ([c-t-t](https://llvm-compile-time-tracker.com/compare.php?from=6d48d45c130293452e398cda29ea7d348f384034&to=28d0af9d63358e16f6d3e627dce30029d9c2b33a&stat=instructions:u))

https://github.com/llvm/llvm-project/pull/206018