[Support] No longer require flushing raw_string_ostream

Since 65b13610, raw_string_ostream
has been unbuffered by default, making .flush() a no-op. This diff
formalizes this by no longer .flush()ing in the .str() method or
the destructor. .str() has been marked as "consider removing", since
its primary use case used to be making .flush()+access a one-liner,
and it also has issues such as preventing NRVO/implicit move when used
in return statements.

Differential Revision: https://reviews.llvm.org/D115421