[Coverage] Prevent detection of false instantiations in case of macro expansion.

The root of the problem was that findMainViewFileID(File, Function)
could return some ID for any given file, even though that file
was not the main file for that function.

This patch ensures that the result of this function is conformed
with the result of findMainViewFileID(Function).

Differential Revision: http://reviews.llvm.org/D18787

llvm-svn: 266436