AutoGen: Fix needless compilation of mocs_compilation.cpp

For an AUTOMOC'ed target foo, when depfiles are used, there are the
following dependencies:

foo_autogen -> foo_autogen/timestamp
foo_autogen/mocs_compilation.cpp -> foo_autogen/timestamp

The first is used to trigger AUTOGEN.
The second erroneously triggers a recompilation of
mocs_compilation.cpp when any source file of target foo is touched.

Remove the latter dependency and replace it with an order-only
dependency from foo_autogen/mocs_compilation.cpp to

That is achieved by making mocs_compilation.cpp or its
per-configuration variants byproduct(s) of the timestamp file.

Fixes: #22338
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