Reapply [Coverage] Fix an assertion failure if the definition of an unused function...

Reapply [Coverage] Fix an assertion failure if the definition of an unused function spans multiple files.

We have an assertion failure if, for example, the definition of an unused
inline function starts in one macro and ends in another. This patch fixes
the issue by finding the common ancestor of the start and end locations
of that function's body and changing the locations accordingly.

Thanks to NAKAMURA Takumi for helping with fixing the test failure on Windows.

Differential Revision: http://reviews.llvm.org/D20997

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271995 91177308-0d34-0410-b5e6-96231b3b80d8