[XRay] cache symbolized function names for a repeatedly queried function ID
Processing 2 GB XRay traces with "llvm-xray convert -symbolize" needs to
go over each trace record and symbolize the function name refered to by
its ID. Currently this happens by asking the LLVM symbolizer code every
single time. A simple cache can save around 30 minutes of processing of
llvm-xray's resident memory usage increased negligibly with this cache.
Differential Revision: https://reviews.llvm.org/D43896