[llvm] [BOLT] Store FileSymRefs in a multimap (PR #98992)

[Amir Ayupov via llvm-commits]

aaupov wrote:

> LGTM. Is this map live for the entire execution? Is there a reported memory increase? Thanks!

Yes, currently it is live throughout the execution, even though it can be freed at the end of `discoverFileObjects`. With a large binary test, there's no noticeable change in peak RSS: 30.07Gb vs 30.09Gb. 

https://github.com/llvm/llvm-project/pull/98992