[llvm] [Dwarf] Don't crash in dwarf::isC() on vendor-defined languages (PR #223739)

[James Henderson via llvm-commits]

================
@@ -0,0 +1,26 @@
+; DW_LANG_lo_user and DW_LANG_hi_user are enumerated by dwarf::isC(), but the
+; vendor-defined codes between them are not, and used to fall through to the
+; trailing llvm_unreachable. Emitting a prototyped subprogram in such a
----------------
jh7370 wrote:

Nit: "used to fall through to a trailing llvm_unreachable"

https://github.com/llvm/llvm-project/pull/223739