Fix memory leak exposed by r304726.

When giving a ContentCache a null buffer, ignore the DoNotFree flag rather than
inheriting it onto whatever buffer we end up using for the file. Also ensure
that the main buffer is properly destroyed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304740 91177308-0d34-0410-b5e6-96231b3b80d8