[libc++] Add an option to disable wide character support in libc++

Some embedded platforms do not wish to support the C library functionality
for handling wchar_t because they have no use for it. It makes sense for
libc++ to work properly on those platforms, so this commit adds a carve-out
of functionality for wchar_t.

Unfortunately, unlike some other carve-outs (e.g. random device), this
patch touches several parts of the library. However, despite the wide
impact of this patch, I still think it is important to support this
configuration since it makes it much simpler to port libc++ to some
embedded platforms.

Differential Revision: https://reviews.llvm.org/D111265

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.assign/move.pass.cpp

@@ -38,6 +38,7 @@ int main(int, char**)
         buf = move(buf1);
         assert(buf.str() == "testing");
     }
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {
         std::wstringbuf buf1(L"testing");
         std::wstringbuf buf;
@@ -56,6 +57,7 @@ int main(int, char**)
         buf = move(buf1);
         assert(buf.str() == L"testing");
     }
+#endif // TEST_HAS_NO_WIDE_CHARACTERS
 
   return 0;
 }