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

[via cfe-commits]

cjc0013 wrote:

Updated to separate GMF internal-linkage identity from the ODR-check policy.

- Added a dedicated `-f[no-]modules-unique-gmf-internal-linkage` option, enabled by default.
- AST deserialization and Itanium mangling now both use that option; neither depends on `-fskip-odr-check-in-gmf`.
- For qualifying GMF internal-linkage functions, `findExisting` now returns `false` directly.
- The regression crosses the controls in both directions: unique GMF identity with ODR checking enabled, and ordinary identity with ODR checking skipped.
- Moved the documentation out of the ODR-check section and documented the independent opt-out.

Validation:

- `ninja -C /root/llvm-build-195661 -j4 clang`: passed
- focused GMF regression: 1 passed
- adjacent C++20 module tests: 4 passed
- changed-line clang-format and `git diff --check`: clean


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