[clang] [modules] Avoid redefinition errors when access modular header as non-modular. (PR #214345)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Does this depends on https://github.com/llvm/llvm-project/commit/3ce0df7ca8027229618de9802eeffe1f0023bf74 ? I am going to revert that due to the report in https://github.com/llvm/llvm-project/issues/214091

https://github.com/llvm/llvm-project/pull/214345