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

https://github.com/llvm/llvm-project/pull/95482 is a reland of
https://github.com/llvm/llvm-project/pull/88024.
https://github.com/llvm/llvm-project/pull/95482 keeps indexing memory
usage reasonable by using unordered_map and doesn't make other changes
to originally reviewed code.

While discussing possible ways to minimize indexing memory usage, Teresa
asked whether I need `ExportSetTy` as a map or a set is sufficient. This
PR implements the idea. It uses a set rather than a map to track exposed
ValueInfos.

Currently, `ExportLists` has two use cases, and neither needs to track a
ValueInfo's import/export status. So using a set is sufficient and
correct.
1) In both in-process and distributed ThinLTO, it's used to decide if a
function or global variable is visible [1] from another module after importing
creates additional cross-module references.
     * If a cross-module call edge is seen today, the callee must be visible
       to another module without keeping track of its export status already.
       For instance, this [2] is how callees of direct calls get exported.
2) For in-process ThinLTO [3], it's used to compute lto cache key.
     * The cache key computation already hashes [4] 'ImportList' , and 'ExportList' is
        determined by 'ImportList'. So it's fine to not track 'import type' for export list.

[1] https://github.com/llvm/llvm-project/blob/66cd8ec4c08252ebc73c82e4883a8da247ed146b/llvm/lib/LTO/LTO.cpp#L1815-L1819
[2] https://github.com/llvm/llvm-project/blob/66cd8ec4c08252ebc73c82e4883a8da247ed146b/llvm/lib/LTO/LTO.cpp#L1783-L1794
[3] https://github.com/llvm/llvm-project/blob/66cd8ec4c08252ebc73c82e4883a8da247ed146b/llvm/lib/LTO/LTO.cpp#L1494-L1496
[4] https://github.com/llvm/llvm-project/blob/b76100e220591fab2bf0a4917b216439f7aa4b09/llvm/lib/LTO/LTO.cpp#L194-L222

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