[clang] Expand invalid PCM diagnostic (#69489)

Summary:

When a PCM file is loaded, it can go wrong in various ways. The current
diagnostic only produces the name of the malformed PCM, not why it is
malformed. Expand the diagnostic to display what went wrong!

There is only one call site for this diagnostic, and it already passes
the error message:


https://github.com/llvm/llvm-project/blob/main/clang/lib/Serialization/ASTReader.cpp#L4763-L4764



Test Plan:

The modified LIT test.

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Co-authored-by: Nuri Amari <nuriamari@fb.com>