[clang] [llvm] demangle function names in trace files (PR #87626)

[Reid Kleckner via cfe-commits]

rnk wrote:

I think I'll leave this change in and file an issue about it. A 10% compile time regression with `-ftime-trace` isn't great, but it doesn't feel revert-worthy.

https://github.com/llvm/llvm-project/pull/87626