[streambuf] Added call to traits_type::copy to common case in xsgetn()

Patch by Laman Sole <[email protected]>, Sebastian Pop
<[email protected]>, Aditya Kumar <[email protected]>

Differential Revision: http://reviews.llvm.org/D21103

llvm-svn: 272401
1 file changed