Remove and forbid raw_svector_ostream::flush() calls.

Remove and forbid raw_svector_ostream::flush() calls.
After r244870 flush() will only compare two null pointers and return,
doing nothing but wasting run time. The call is not required any more
as the stream and its SmallString are always in sync.

Thanks to David Blaikie for reviewing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244928 91177308-0d34-0410-b5e6-96231b3b80d8