Teach SourceManager::getLocation() how to cope with a source file

whose inode has changed since the file was first created and that is
being seen through a different path name (e.g., due to symlinks or
relative path elements), such that its FileEntry pointer doesn't match
a known FileEntry pointer. Since this requires a system call (to
stat()), we only perform this deeper checking if we can't find the
file by comparing FileEntry pointers.

Also, add a micro-optimization where we don't bother to compute line
numbers when given the location (1, 1). This improves the
efficiency of clang_getLocationForOffset().
 

llvm-svn: 124800