Extend the source manager with the ability to override the contents of

Extend the source manager with the ability to override the contents of
files with the contents of an arbitrary memory buffer. Use this new
functionality to drastically clean up the way in which we handle file
truncation for code-completion: all of the truncation/completion logic
is now encapsulated in the preprocessor where it belongs
(<rdar://problem/7434737>).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90300 91177308-0d34-0410-b5e6-96231b3b80d8