With aggressive ICF, it's possible to have different local symbols
(under different FILE symbols) to be mapped to the same address.
FileSymRefs only keeps a single SymbolRef per address, which prevents
fragment matching from finding the correct symbol to perform parent
function lookup.
Work around this issue by switching FileSymRefs to a multimap. In
future, uses of FileSymRefs can be replaced with SortedSymbols which
keeps essentially the same information.
Test Plan: added ambiguous_fragment.test
Reviewers: dcci, ayermolo, maksfb, rafaelauler
Reviewed By: rafaelauler
Pull Request: https://github.com/llvm/llvm-project/pull/98992
3fe50b6dde