Allow calling wxStrchr() with a narrow string and wide character.

Allow calling wxStrchr() with a narrow string and wide character.

Calls to wxStrchr(char-string, wide-char) would previously fail if wide
character couldn't be converted to a single character in the current locale
encoding. Change it to simply return NULL in this case as it's a safe and
useful generalization: a narrow string will never contain a wide character not
representable in the current locale.

Add wxUniChar::GetAsChar() to help with implementing this.

Closes #11487.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62738 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775