Have SourceManager::getLocForEndOfFile() point at the "EOF" location of the FileID.

This fixes a crash due to SourceManager::getLocForEndOfFile() returning an off-by-one location
when the the FileID is for an empty file.

rdar://13803893

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181285 91177308-0d34-0410-b5e6-96231b3b80d8