c++: C++17 constexpr static data member linkage [PR99901]

c++: C++17 constexpr static data member linkage [PR99901]

C++17 makes constexpr static data members implicitly inline variables.  In
C++14, a subsequent out-of-class declaration is the definition.  We want to
continue emitting a symbol for such a declaration in C++17 mode, for ABI
compatibility with C++14 code that wants to refer to it.

Normally I'd distinguish in- and out-of-class declarations by looking at
DECL_IN_AGGR_P, but we never set DECL_IN_AGGR_P on inline variables.  I
think that's wrong, but don't want to mess with it so close to release.
Conveniently, we already have a test for in-class declaration earlier in the
function.

gcc/cp/ChangeLog:

	PR c++/99901
	* decl.c (cp_finish_decl): mark_needed an implicitly inline
	static data member with an out-of-class redeclaration.

gcc/testsuite/ChangeLog:

	PR c++/99901
	* g++.dg/cpp1z/inline-var9.C: New test.