[libc++] Explicitly initialize std::nothrow

When building on Windows without libc++abi, this change fixes a build error of the form:

    src/new.cpp(38,17):  error: chosen constructor is explicit in copy-initialization
    const nothrow_t nothrow = {};
    include/vcruntime_new.h(53,22):  note: explicit constructor declared here
                explicit nothrow_t() = default;

Differential Revision: https://reviews.llvm.org/D57351


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@352648 91177308-0d34-0410-b5e6-96231b3b80d8