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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216614 91177308-0d34-0410-b5e6-96231b3b80d8