[clang] [C++][Modules] Don't insert `#include` before GMF when `-include` used (PR #212533)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

LGTM. But given I am not familair with the lexer part, I'll leave the formal approval to others.

https://github.com/llvm/llvm-project/pull/212533