c++/modules: Validate external linkage definitions in header units [PR116401]

c++/modules: Validate external linkage definitions in header units [PR116401]

[module.import] p6 says "A header unit shall not contain a definition of
a non-inline function or variable whose name has external linkage."

This patch implements this requirement, and cleans up some issues in the
testsuite where this was already violated.  To handle deduction guides
we mark them as inline, since although we give them a definition for
implementation reasons, by the standard they have no definition, and so
we should not error in this case.

	PR c++/116401

gcc/cp/ChangeLog:

	* decl.cc (grokfndecl): Mark deduction guides as 'inline'.
	* module.cc (check_module_decl_linkage): Implement checks for
	non-inline external linkage definitions in headers.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/macro-4_c.H: Add missing 'inline'.
	* g++.dg/modules/pr106761.h: Likewise.
	* g++.dg/modules/pr98843_b.H: Likewise.
	* g++.dg/modules/pr99468.H: Likewise.
	* g++.dg/modules/pragma-1_a.H: Likewise.
	* g++.dg/modules/tpl-ary-1.h: Likewise.
	* g++.dg/modules/hdr-2.H: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>