[libc++] Diagnose when nullptrs are passed to string APIs (#122790)

This allows catching misuses of APIs that take a pointer to a
null-terminated string.

runtimes/cmake/Modules/WarningFlags.cmake

@@ -25,6 +25,7 @@ function(cxx_add_warning_flags target enable_werror enable_pedantic)
       -Wformat-nonliteral
       -Wzero-length-array
       -Wdeprecated-redundant-constexpr-static-def
+      -Wno-nullability-completeness
       )
 
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")