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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

(The merge interface has been tweaked to make me click past a scary-looking "you probably shouldn't be checking this" warning to merge before CI checks complete... so I'll just wait for the checks :laughing: )

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