Report ambiguous uses of .md attributes

Report ambiguous uses of .md attributes

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