[llvm-cov] Read in function names for filtering from a text file.

Summary: Add a -name-whitelist option, which behaves in the same way as -name, but it reads in multiple function names from the given input file(s).

Reviewers: vsk

Reviewed By: vsk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37111

llvm-svn: 312227