The variable was only set by the GNU-like compiler modules, so AUTOMOC
never generated a moc_predefs.h for compilers targeting the MSVC ABI
and moc parsed headers without the compiler's predefined macros.
MSVC dumps its predefined macros with '/PD', which needs the conforming
preprocessor. Require VS 2019 16.8, because 16.7 and older ignore the
flag with warning D9002 and emit no macros at all. clang-cl takes '-dM'
through '-Xclang'. Both also echo the preprocessed source, so feed them
an empty translation unit. clang with the GNU driver is excluded from
'Compiler/GNU' as well when it simulates MSVC, so give it the usual
'-dM -E' command there.
Fixes: #28093
