[Coverage] Speed up file-based queries for coverage info, NFC

Speed up queries for coverage info in a file by reducing the amount of
time spent determining whether a function record corresponds to a file.

This gives a 36% speedup when generating a coverage report for `llc`.
The reduction is entirely in user time.

rdar://54758110

Differential Revision: https://reviews.llvm.org/D67575

llvm-svn: 372025