Fix llvm.ident exploding when linking many bitcode files in libclc. This
should de-duplicate other named metadata as well, e.g.
opencl.spir.version and opencl.ocl.version.
This PR is a re-submit of https://reviews.llvm.org/D20582 (authored by
Matt Arsenault) with update that only checks MD node pointer for
duplication according to review comment in that PR.
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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>
a6e870d0b6