Make the private.h header compatible with Clang.

The include/wx/msw/private.h header contains a violation of the C++
standards that MSVC tolerates but Clang/LLVM does not.

WinBase.h contains the definition:
    #define INVALID_HANDLE_VALUE ((HANDLE)(LONG_PTR)-1)

Which wxWidgets uses in:
    template <wxUIntPtr INVALID_VALUE =
        (wxUIntPtr)INVALID_HANDLE_VALUE>

The (effective) reinterpret_cast-ing of INVALID_HANDLE_VALUE in
WinBase.h means that the define isn't actually usable as a constant
expression for the template parameter.

Clang has indicated they have no intention of adding an MSVC
compatibility workaround for this problem. See LLVM bug 12116:
https://bugs.llvm.org/show_bug.cgi?id=12116

Given this, rework the wxWidgets header to remove the default
template parameter value (it is not used anywhere).
With this change, it's possible to compile wxWidgets applications
using Clang/LLVM (5.0) under MSVC.

include/wx/msw/private.h

@@ -153,7 +153,7 @@ extern LONG APIENTRY
 #endif
 
 // close the handle in the class dtor
-template <wxUIntPtr INVALID_VALUE = (wxUIntPtr)INVALID_HANDLE_VALUE>
+template <wxUIntPtr INVALID_VALUE>
 class AutoHANDLE
 {
 public: