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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> Does this depends on [3ce0df7](https://github.com/llvm/llvm-project/commit/3ce0df7ca8027229618de9802eeffe1f0023bf74) ? I am going to revert that due to the report in #214091

Nope, these changes are independent.

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