QVariant: make the HasIsNullMethod check work across all compilers (maybe)

Currently, we have a C++11 version that requires Q_COMPILER_DECLTYPE
support, and can deal with final classes, and a C++98 version that
doesn't require any C++11 features, but fails on final classes.

What we're missing is a version that works for MSVC v8 and v9
(2005 and 2008), which sport the 'sealed' non-standard keywords
but lack decltype support. So far, we tried to solve the problem
by making class-level final special (Q_DECL_FINAL_CLASS), not
defining that macro for these two compilers, even though we did
define Q_DECL_FINAL, the method-level keyword.

This new formulation, taken from
  http://stackoverflow.com/a/9655327/134841
supposedly supports all compilers with a minor #ifdef for
MSVC which doesn't like applying sizeof() the way we do.

However, testing has shown this to blow up on OSX.

So we use the less intrusive approach: add this variant as
a third version, only used by VC 2005 and 2008.

Change-Id: If1945f8a6e9ed36cb68212fa781d5e29eb2a082d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

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