[XRay] cache symbolized function names for a repeatedly queried function ID

[XRay] cache symbolized function names for a repeatedly queried function ID

Summary:
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
that trace.

llvm-xray's resident memory usage increased negligibly with this cache.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43896

llvm-svn: 326407