DI: LLVM schema change: fold constants into string

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@218913 91177308-0d34-0410-b5e6-96231b3b80d8