[libc++] Explicitly initialize std::nothrow

[thomasanderson]

Summary

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;

[EricWF]

src/new.cpp

37

What about const nothrow_t nothrow{}; instead? That way we can leave the copy constructor out of it.

src/new.cpp

 #   include "support/runtime/new_handler_fallback.ipp"
 # endif
 #endif
 
 namespace std
 {
 
 #ifndef __GLIBCXX__
-const nothrow_t nothrow = {};
+const nothrow_t nothrow{};
 #endif
 
 #ifndef LIBSTDCXX
 
 void
 __throw_bad_alloc()
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS