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

Patch by Laman Sole <laxman.g@partner.samsung.com>, Sebastian Pop
<s.pop@samsung.com>, Aditya Kumar <aditya.k7@samsung.com>

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

llvm-svn: 272401