gccrs: Avoid malformed attribute conversion

gccrs: Avoid malformed attribute conversion

Add location for malformed attribute errors and avoid processing of
malformed attributes.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (AttrInputMetaItemContainer::separate_cfg_attrs):
	Avoid malformed attributes.
	* util/rust-attributes.cc (AttributeChecker::visit): Change location.

gcc/testsuite/ChangeLog:

	* rust/compile/attr_malformed_path.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>