[clang] [clang] Disambiguate GMF internal functions across partitions (PR #219238)

[via cfe-commits]

cjc0013 wrote:

I reworked this along those lines.

The partition-specific mangling and dedicated flag are removed. Internal-linkage functions from global module fragments now retain their ordinary mangling. Matching serialized ODR hashes merge as one entity, while mismatches are routed through Clang's existing deferred function-ODR diagnostic after the lazy bodies are loaded.

The regression uses `extern "C"` `static inline` functions and covers both cases with `-fskip-odr-check-in-gmf`: identical bodies produce one helper definition used by both callers, while different bodies fail with the existing different-definitions diagnostic.

Validation:

- affected source compilation and clang relink: passed
- focused GMF regression: 1 passed
- adjacent C++20 module tests: 4 passed
- changed-line formatting and `git diff --check`: clean


https://github.com/llvm/llvm-project/pull/219238