[clang][Modules] Keep current-TU global-module-fragment internal-linkage functions viable in overload resolution (#210825)

Fixes #210822

A `static` (or anonymous-namespace) function declared in the global
module fragment of the current TU is usable within the module unit
([module.global.frag]/p1), but `Sema::AddOverloadCandidate` dropped it
from the overload set when `Function->isInAnotherModuleUnit()` misfired
— which it does when the enclosing template is instantiated after the
GMF closes (pending instantiations run in
`ActOnEndOfTranslationUnitFragment`, before the named module is in
scope). The candidate was removed with the hidden
`ovl_fail_module_mismatched` kind, so the error had no candidate notes.

The existing carve-out (#104701) only exempted inline /
implicitly-instantiated GMF functions. This also exempts
internal-linkage functions from the current TU's GMF
(`isFromGlobalModule() && !isFromASTFile()`); genuinely imported
internal-linkage functions stay subject to the rule.

`clang/test/Modules` and `clang/test/CXX/module` show no new failures;
the `expose-static-inline-from-gmf-*` tests still pass. Adds a
regression test.

This change was developed with substantial assistance from an AI tool
(Claude Code). The AI carried out the investigation and root-cause
analysis, wrote the code change and the regression test, and drafted
this description.