NFCI: Simplify SourceManager::translateFile by removing code path that should never be taken

I noticed that SourceManager::translateFile has code that doesn't really make sense.
In particular, if it fails to find a FileID by comparing FileEntry * values, it tries to
look through files that have the same filename, to see if they have a matching inode to try to
find the right FileID. However, the inode comparison seem redundant, as Clang's FileManager
already deduplicates FileEntry * values by inode.
Thus the comparisons between inodes should never actually succeed, and the comparison between FileEntry * values should be sufficient here.

Differential Revision: https://reviews.llvm.org/D65481

llvm-svn: 369585