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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219010 91177308-0d34-0410-b5e6-96231b3b80d8

test/Linker/debug-info-version-b.ll

@@ -5,6 +5,6 @@
 !llvm.dbg.cu = !{!1}
 
 !0 = metadata !{i32 2, metadata !"Debug Info Version", i32 42}
-!1 = metadata !{i32 589841, metadata !2, i32 12, metadata !"clang", metadata !"I AM UNEXPECTED!"} ; [ DW_TAG_compile_unit ]
+!1 = metadata !{metadata !"0x11\0012\00clang\000\00", metadata !"I AM UNEXPECTED!"} ; [ DW_TAG_compile_unit ]
 !2 = metadata !{metadata !"b.c", metadata !""}
 !3 = metadata !{}