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.

gcc/cp/decl.cc

@@ -8180,6 +8180,12 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  return;
 	}
       cp_apply_type_quals_to_decl (cp_type_quals (type), decl);
+
+      /* Update the type of the corresponding TEMPLATE_DECL to match.  */
+      if (DECL_LANG_SPECIFIC (decl)
+	  && DECL_TEMPLATE_INFO (decl)
+	  && DECL_TEMPLATE_RESULT (DECL_TI_TEMPLATE (decl)) == decl)
+	TREE_TYPE (DECL_TI_TEMPLATE (decl)) = type;
     }
 
   if (ensure_literal_type_for_constexpr_object (decl) == error_mark_node)

gcc/testsuite/g++.dg/modules/auto-3.h

@@ -0,0 +1,10 @@
+template<class>
+struct __tree_barrier {
+  static const auto __phase_alignment_1 = 0;
+
+  template<class>
+  static const auto __phase_alignment_2 = 0;
+};
+
+template<class>
+inline auto __phase_alignment_3 = 0;

gcc/testsuite/g++.dg/modules/auto-3_a.H

@@ -0,0 +1,4 @@
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#include "auto-3.h"

gcc/testsuite/g++.dg/modules/auto-3_b.C

@@ -0,0 +1,4 @@
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+#include "auto-3.h"
+import "auto-3_a.H";