[libc++abi] Fix C++ demangling for _BitInt type (#143466)

The front-end expects _BitInt to be available for substitution; ensure DB<n> is
added to Subs in ItaniumDemangle.h. Also add a test case to libc++abi and
sync the files to llvm/include/llvm.

GitOrigin-RevId: 20f56d140909a01c74e9981835373eaab6021af9
2 files changed