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

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@272401 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx