c++: modules and non-dependent auto deduction

c++: modules and non-dependent auto deduction

The modules streaming code seems to rely on the invariant that a
TEMPLATE_DECL and its DECL_TEMPLATE_RESULT have the same TREE_TYPE.
But for a non-dependent VAR_DECL with deduced type, the two TREE_TYPEs
end up diverging: cp_finish_decl deduces the type of the initializer
ahead of time and updates the TREE_TYPE of the VAR_DECL, but neglects to
update the corresponding TEMPLATE_DECL as well, which leads to a
"conflicting global module declaration" error for each of the
__phase_alignment decls in the below testcase (and for the xtreme-header
tests if we try including <barrier>).

This patch makes cp_finish_decl update the TREE_TYPE of the corresponding
TEMPLATE_DECL so that the invariant is maintained.

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): After updating the deduced type of a
	VAR_DECL, also update the corresponding TEMPLATE_DECL if there
	is one.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/auto-3.h: New test.
	* g++.dg/modules/auto-3_a.H: New test.
	* g++.dg/modules/auto-3_b.C: New test.