Fix wxMSW build with WXWIN_COMPATIBILITY_2_6==1.

Avoid conflict between wxMSW symbol defined in wx/platinfo.h in this build
configuration and wxMSW namespace by renaming the latter to wxMSWImpl, which
is also consistent with the namespace name used elsewhere.

Closes #15624.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75103 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775