Saleem Abdulrasool: If errno is defined as volatile int, the qualifier differences can cause
template typename deductions on swap<> (used in string.cpp).  Use
decltype(errno) to replicate the type and qualifier information for holding the
errno value.  Because errno is expected to be assignable, there is no need to
use typename std::remove_const<decltype(errno)>::type to hold the value.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@173172 91177308-0d34-0410-b5e6-96231b3b80d8