c++: Clear is_unbraced_* when parsing declaration_seq_opt [PR114917]

c++: Clear is_unbraced_* when parsing declaration_seq_opt [PR114917]

Currently we incorrectly retain "in_unbraced_linkage_specification_p"
and "in_unbraced_export_declaration_p" when parsing a (braced)
declaration-seq.  This patch ensures that we clear these flags before
parsing the toplevel declarations.

Strictly speaking we don't need to save and restore the flags around the
parsing because there's currently no way to provide new declarations
within the unbraced context after the closing brace, but this patch does
it anyway in case this ever changes and for consistency with other
places that these flags are adjusted.

	PR c++/114917

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_declaration_seq_opt): Clear
	parser->in_unbraced_* flags when parsing toplevel declarations.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/export-5_a.C: New test.
	* g++.dg/modules/export-5_b.C: New test.
	* g++.dg/parse/linkage4.C: New test.

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