Revert "Revert "DI: Fold constant arguments into a single MDString""

This reverts commit r218918, effectively reapplying r218914 after fixing
an Ocaml bindings test and an Asan crash.  The root cause of the latter
was a tightened-up check in `DILexicalBlock::Verify()`, so I'll file a
PR to investigate who requires the loose check (and why).

Original commit message follows.

--

This patch addresses the first stage of PR17891 by folding constant
arguments together into a single MDString.  Integers are stringified and
a `\0` character is used as a separator.

Part of PR17891.

Note: I've attached my testcases upgrade scripts to the PR.  If I've
just broken your out-of-tree testcases, they might help.
Revert "Revert "DI: LLVM schema change: fold constants into string""

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

--

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

Part of PR17891.