[llvm] [ThinLTO] Use a set rather than a map to track exported ValueInfos. (PR #97360)

[Mingming Liu via llvm-commits]

minglotus-6 wrote:

@teresajohnson @jvoung I'm also experimenting with a change make `ExportSetTy` an unordered_set rather than a `DenseSet` (by implementing `std::hash` for `ValueInfo`). If it reduces the memory usage of indexing across multiple binaries (because the distribution of exported values per module shows most container size is smaller than 64), I'm considering that change as a follow up of this one. 

https://github.com/llvm/llvm-project/pull/97360