Fix compilation with Xcode projects (type_traits).

The changes to type_traits headers detection with Clang broke Clang use
with the Xcode projects. Fix this by not setting HAVE_* macros in
config_xcode.h for Clang and letting defs.h detect the features.

See #15915.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76017 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775