Clean up iostream creation in preparation for conditionalizing streams.

Interleave the code for narrow and wide character streams. This makes it
more obvious that the two pieces of code are identical. Furthermore, it
makes it easier to conditionally compile support for certain streams, as
less #ifdef blocks are needed.

Differential Revision:	http://reviews.llvm.org/D8342
Reviewed by:	marshall

llvm-svn: 232516