Avoid using "do { ... } while ( wxFalse )" pseudo-loop.

Avoid using "do { ... } while ( wxFalse )" pseudo-loop.

This loop can't be optimized away by the compiler because wxFalse is an extern
variable which can't be known to be always false. Additionally, this creates
many false positives from Coverity as it assumes that the loop can be executed
more than once.

Define wxSTATEMENT_MACRO_BEGIN/END macros abstracting the exact solution used
and replace wxFalse with "(void)0, 0" for now as this seems to placate MSVC
(which warns about using a bare "0" as a condition) while still allowing the
loop to be completely optimized away.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73101 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775