C++ PATCH to mark_decl_instantiated

[Jason Merrill]

In the discussion of PR 59469, Honza said that the C++ front end should 
be setting forced_by_abi on explicitly instantiated decls.  This patch 
does that, though unfortunately it does not seem to be sufficient to fix 
the bug.

Tested x86_64-pc-linux-gnu, applying to trunk.
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