DI: LLVM schema change: fold constants into string

Update debug info testcases for the LLVM metadata schema change in
r219010 to fold metadata constant operands into a single `MDString`.

Part of PR17891.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@219019 91177308-0d34-0410-b5e6-96231b3b80d8