c++/modules: Check linkage of structured binding decls

When looking at PR c++/118513 I noticed that we don't currently check
the linkage of structured binding declarations in modules.  This patch
adds those checks, and corrects decl_linkage to properly recognise
structured binding declarations as potentially having linkage.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_decomposition_declaration): Check linkage
	of structured bindings in modules.
	* tree.cc (decl_linkage): Structured bindings don't necessarily
	have no linkage.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/export-6.C: Add structured binding tests.
	* g++.dg/modules/hdr-2.H: Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>