[clang] [llvm] [clang][LLVM Demangler] Add an assertion that validates that all mang… (PR #111391)

[Viktoriia Bakalova via cfe-commits]

VitaNuo wrote:

> Since this is being done as an assert, this is something that really only applies during debug mode. I dont think this compiler flag makes sense because of it.

The reason I am adding a flag on top of an assert is that there are unfortunately still many mangled names produced by `clang` that cannot be demangled. E.g., many of the existing codegen lit tests (`clang/test/CodeGenCXX/*`) fail in assertion-enabled builds without the flag. This is potentially disruptive to development workflows. The flag can be removed once the LLVM demangler can handle all the mangled names `clang` produces.

https://github.com/llvm/llvm-project/pull/111391