[BOLT] Store FileSymRefs in a multimap

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 s

[BOLT] Store FileSymRefs in a multimap

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

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