This patch reports an error if the .md file has an unscoped
attribute that maps to more than one possible value.

2019-07-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* read-md.h (md_reader::record_potential_iterator_use): Add a
	file_location parameter.
	* read-rtl.c (attribute_use::loc): New field.
	(map_attr_string): Take a file_location parameter.  Report cases
	in which attributes map to multiple distinct values.
	(apply_attribute_uses): Update call accordingly.
	(md_reader::handle_overloaded_name): Likewise.
	(md_reader::apply_iterator_to_string): Likewise.  Skip empty
	nonnull strings.
	(record_attribute_use): Take a file_location parameter.
	Initialize attribute_use::loc.
	(md_reader::record_potential_iterator_use): Take a file_location
	parameter.  Update call to record_attribute_use.
	(rtx_reader::rtx_alloc_for_name): Update call accordingly.
	(rtx_reader::read_rtx_code): Likewise.
	(rtx_reader::read_rtx_operand): Likewise.  Record a location
	for implicitly-expanded empty strings.

From-SVN: r273511