Centralize where we update the source file contents in...

Centralize where we update the source file contents in SourceManager::GetFile() in case APIs are called that don't update the source.

The following functions were the only functions that updates the source file:

SourceManager::File::DisplaySourceLines()
SourceManager::File::FindLinesMatchingRegex()    

But there we API calls that were using the SourceManager::File and asking it questions, like "is line 12 valid" and that might respond incorrectly if the source file had been updated.

<rdar://problem/21269402>

llvm-svn: 243551