[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.

0 files changed, 0 insertions, 0 deletions