Automoc: Add directory-level COMPILE_DEFINITIONS to command line (#14535)

In commit 18412153 (Refactor cmTarget::GetCompileDefinitions...,
2013-06-06) cmQtAutomoc was refactored to get all compile definitions
from the target instead of separately asking the target and querying the
directory-level COMPILE_DEFINITIONS property value.  While the
generation process does integrate directory-level COMPILE_DEFINITIONS
into the target definitions, this did not happen until after Automoc
targets are constructed.  Therefore the commit regressed use of
directory-level definitions in Automoc targets.

Factor the definition finalization logic out from the
cmGlobalGenerator::CreateGeneratorTargets method into a new
cmGlobalGenerator::FinalizeTargetCompileDefinitions method and call it
before constructing Automoc targets.  This will place the
directory-level definitions into the target in time to use them for the
Automoc targets.

Extend the QtAutomoc test to cover this case.
Merge branch 'fix-automoc-compile-definitions' into next