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.
