[llvm] [SPGO] Use std::hash instead of MD5 to avoid run time regression in llvm-profgen (PR #180581)

[via llvm-commits]

WenleiHe wrote:

I wonder how much cost is still in MD5 hash after this patch? In other words, if we replace MD5 with say xxhash for FunctionId in general, would it speed things up more for compiler and llvm-profgen? 

https://github.com/llvm/llvm-project/pull/180581