[gcov] Sort file info before printing it

The order in which GCOV file info is printed depends on the string hash
function. This makes some GCOV tests brittle, because the tests must be
updated whenever the hash function changes.

Sort the filenames before printing out the file info to solve the
problem. This should be relatively cheap.

Differential Revision: https://reviews.llvm.org/D32512

llvm-svn: 301371