[flang][OpenMP] Fix declare reduction lookup for USE...ONLY imports

CheckSymbolSupportsType walked every module in the global scope to find
declare-reduction declarations. That accepted reductions from modules
that were never USE'd, or were excluded via USE...ONLY, and it still
rejected some valid imports such as a renamed operator.

Replace the global scan with FindUserReduction(), which resolves the
reduction the way name resolution resolves the operator. It checks a
directly visible reduction first, then follows the operator's USE
associations and merged-generic sources to the declaring modules,
re-deriving the source module's mangled name for renamed operators. The
search recurses through re-exporting (facade) modules and is type-aware,
so an operator that carries reductions for several types resolves to the
one supporting the requested type. A locally declared reduction is
authoritative and shadows reductions reachable through the operator.

Consolidate the duplicated GetReductionFortranId() (formerly static in
both resolve-names.cpp and mod-file.cpp) into a shared utility, fixing a
latent bug where defined operators were not correctly reverse-mapped.

This corrects the semantic check only. The local lowering ICE for a
user-defined operator declare reduction was fixed separately in
https://github.com/llvm/llvm-project/issues/204299. The cross-module
cases this change additionally accepts (USE-associated, renamed, or
merged operators) now emit a clean "not yet implemented" diagnostic in
lowering. Completing their lowering is a follow-up.

Fixes https://github.com/llvm/llvm-project/issues/200300.

Assisted-by: Claude Opus 4.8.