Fix 62 harmless but annoying Clang warnings in wxOSX build.

Fix 62 harmless but annoying Clang warnings in wxOSX build.

Clang warns about using letters and digits in a switch on wxKeyCode enum which
doesn't include them as elements. This is generally useful but really annoying
in this case, especially due to the sheer number of warnings, so disable it
using Clang-specific pragma.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71518 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775