Overload SourceManager::overrideFileContents so that unconditionally passing...

Overload SourceManager::overrideFileContents so that unconditionally passing ownership is explicitly done using unique_ptr.

Only those callers who are dynamically passing ownership should need the
3 argument form. Those accepting the default ("do pass ownership")
should do so explicitly with a unique_ptr now.

llvm-svn: 216614