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

[Reid Kleckner via llvm-commits]

rnk wrote:

Are we sure we want this behavior? Demangling is really expensive. A user noticed a significant (10%) compile time regression from this change. We could skip the demangling and do it offline. It's also worth pointing out that we have to redo this work every time we optimize the same function over and over. There is a fair amount of room for improvement in efficiency here.

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