c++: unnecessary instantiation of constexpr var [PR99130]

c++: unnecessary instantiation of constexpr var [PR99130]

Here the use of 'value' from within an unevaluated context causes us
to overeagerly instantiate it, via maybe_instantiate_decl called from
mark_used, despite the use occurring in a context that doesn't require
a definition.

This seems to only affect constexpr variable specializations, though
we used to have the same issue for constexpr function specializations
until r6-1309-g81371eff9bc7ef made us delay their instantiation until
necessary during constexpr evaluation.

This patch expands upon the r6-1309 fix to make mark_used avoid
unnecessarily instantiating constexpr variable specializations too,
by pulling out from maybe_instantiate_decl the condition

  (decl_maybe_constant_var_p (decl)
   || (TREE_CODE (decl) == FUNCTION_DECL
       && DECL_OMP_DECLARE_REDUCTION_P (decl))
   || undeduced_auto_decl (decl))

into each of its three callers (including mark_used), removing the
problematic first test from mark_used, and simplifying accordingly.
The net result is that only mark_used is changed because the other two
callers, resolve_address_of_overloaded_function and decl_constant_var_p,
already guard the call appropriately.  (This relaxation of mark_used
seems to be safe because during constexpr evaluation we already take
care to instantiate a constexpr variable as necessary via
decl_constant_value etc).

	PR c++/99130

gcc/cp/ChangeLog:

	* decl2.cc (maybe_instantiate_decl): Adjust function comment.
	Check VAR_OR_FUNCTION_DECL_P.  Pull out the disjunction into ...
	(mark_used): ... here, removing the decl_maybe_constant_var_p
	part of it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-decltype5.C: New test.