Revert "Revert "DI: LLVM schema change: fold constants into string""

This reverts commit r218917, effectively reapplying r218913.  Original
commit message follows.

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Update debug info testcases for an LLVM metadata schema change to fold
metadata constant operands into a single `MDString`.

Part of PR17891.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219011 91177308-0d34-0410-b5e6-96231b3b80d8