[DTLTO][LLD][ELF] Support bitcode members of thin archives (#149425) This patch adds support for bitcode members of thin archives to DTLTO (https://llvm.org/docs/DTLTO.html) in ELF LLD. For DTLTO, bitcode identifiers must be valid paths to bitcode files on disk. Clang does not support archive inputs for ThinLTO backend compilations. This patch adjusts the identifier for bitcode members of thin archives in DTLTO links so that it is the path to the member file on disk, allowing such members to be supported in DTLTO. This patch is sufficient to allow for self-hosting an LLVM build with DTLTO when thin archives are used. Note: Bitcode members of non-thin archives remain unsupported. This will be addressed in a future change. Testing: - LLD lit test coverage has been added to check that the identifier is adjusted appropriately. - A cross-project lit test has been added to show that a DTLTO link can succeed when linking bitcode members of thin archives. For the design discussion of the DTLTO feature, see: #126654.
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