[clang] Insert headers in global module fragment (PR #151624)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

@MythreyaK thanks for the patch!

@JVApen I'll let you take a first crack at reviewing this :) Feel free to flag me if you think it needs an additional pair of eyes.

https://github.com/llvm/llvm-project/pull/151624