Fix bugs in parsing wxLongLong values starting with zeroes.

Fix bugs in parsing wxLongLong values starting with zeroes.

wxCRT_StrtoullBase(), used by wxString::To[U]LongLong(), didn't handle leading
zeroes nor leading 0x correctly: it never auto-detected base 8; didn't ignore
the leading 0 even if base 8 was specified explicitly; didn't recognize "0X"
prefix at all (only "0x").

Fix all these bugs and add test cases for parsing numbers in other bases to
the unit tests.

Closes #14596.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72408 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775