c++: Fix up __builtin_convertvector parsing

c++: Fix up __builtin_convertvector parsing

Jonathan reported on IRC that we don't parse
__builtin_bit_cast (type, val).field
etc.
The problem is that for these 2 builtins we return from
cp_parser_postfix_expression instead of setting postfix_expression
to the cp_build_* value and falling through into the postfix regression
suffix handling loop.

2022-03-26  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_postfix_expression)
	<case RID_BILTIN_CONVERTVECTOR>: Don't
	return cp_build_vec_convert result right away, instead
	set postfix_expression to it and break.

	* c-c++-common/builtin-convertvector-3.c: New test.

(cherry picked from commit 1806829e08f14e4cacacec43d7845cc2dad2ddc8)