[llvm-cov] Cache file status information
Currently, getSourceFile accesses file system to check if two paths are
the same file with a thread lock, which is a huge performance bottleneck
in some cases. Currently, it's accessing file system size(files) * size(files) times.
Thus, cache file status information, which reduces file system access to size(files) times.
When I tested it with two binaries and 16 cpu cores,
it saved over 70% of time.
Binary 1: 56 secs -> 3 secs
Binary 2: 17 hours -> 4 hours
Differential Revision: https://reviews.llvm.org/D97061