[LTO] Introduce a new class ImportIDTable (#106503)

The new class implements a deduplication table to convert import list
elements:

{SourceModule, GUID, Definition/Declaration}

into 32-bit

[LTO] Introduce a new class ImportIDTable (#106503)

The new class implements a deduplication table to convert import list
elements:

{SourceModule, GUID, Definition/Declaration}

into 32-bit integers, and vice versa. This patch adds a unit test but
does not add a use yet.

To be precise, the deduplication table holds {SourceModule, GUID}
pairs. We use the bottom one bit of the 32-bit integers to indicate
whether we have a definition or declaration.

A subsequent patch will collapse the import list hierarchy --
FunctionsToImportTy holding many instances of FunctionsToImportTy --
down to DenseSet<uint32_t> with each element indexing into the
deduplication table above. This will address multiple sources of
space inefficiency.

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