[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.cons/default.pass.cpp

@@ -44,18 +44,20 @@ int main(int, char**)
         std::stringbuf buf;
         assert(buf.str() == "");
     }
-    {
-        std::wstringbuf buf;
-        assert(buf.str() == L"");
-    }
     {
         testbuf<char> buf;
         buf.check();
     }
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    {
+        std::wstringbuf buf;
+        assert(buf.str() == L"");
+    }
     {
         testbuf<wchar_t> buf;
         buf.check();
     }
+#endif
 
 #if TEST_STD_VER >= 11
     {

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

@@ -35,6 +35,7 @@ int main(int, char**)
         std::stringbuf buf(std::move(buf1));
         assert(buf.str() == "testing");
     }
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {
         std::wstringbuf buf1(L"testing");
         std::wstringbuf buf(std::move(buf1));
@@ -50,6 +51,7 @@ int main(int, char**)
         std::wstringbuf buf(std::move(buf1));
         assert(buf.str() == L"testing");
     }
+#endif
 
   return 0;
 }

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

@@ -33,6 +33,7 @@ int main(int, char**)
         std::stringbuf buf("testing", std::ios_base::out);
         assert(buf.str() == "testing");
     }
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
     {
         std::wstringbuf buf(L"testing");
         assert(buf.str() == L"testing");
@@ -45,6 +46,7 @@ int main(int, char**)
         std::wstringbuf buf(L"testing", std::ios_base::out);
         assert(buf.str() == L"testing");
     }
+#endif
 
   return 0;
 }