[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/strings/basic.string/string.cons/string_view_deduction.pass.cpp

@@ -59,6 +59,7 @@ int main(int, char**)
     assert(s1.size() == sv.size());
     assert(s1.compare(0, s1.size(), sv.data(), s1.size()) == 0);
     }
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {
     std::wstring_view sv = L"12345678901234";
     std::basic_string s1{sv, test_allocator<wchar_t>{}};
@@ -69,6 +70,7 @@ int main(int, char**)
     assert(s1.size() == sv.size());
     assert(s1.compare(0, s1.size(), sv.data(), s1.size()) == 0);
     }
+#endif
 #if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
     {
     std::u8string_view sv = u8"12345678901234";