llvm-cov: Use the number of executed functions for the function coverage metric.

This commit fixes llvm-cov's function coverage metric by using the number of executed functions instead of the number of fully covered functions.

Differential Revision: http://reviews.llvm.org/D5196


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218672 91177308-0d34-0410-b5e6-96231b3b80d8