Use file_location for md_reader's ptr_loc

Use file_location for md_reader's ptr_loc

Also make it public, so that clients can use the location for error
reporting.

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

gcc/
	* read-md.h (md_reader::ptr_loc): Moved from read-md.c.
	Use file_location instead of separate fields.
	(md_reader::set_md_ptr_loc): Take a file_location instead of a
	separate filename and line number.
	* read-md.c (ptr_loc): As above.
	(md_reader::copy_md_ptr_loc): Update for new ptr_loc layout.
	(md_reader::fprint_md_ptr_loc): Likewise.
	(md_reader::set_md_ptr_loc): Likewise.  Take a file_location
	instead of a separate filename and line number.
	(md_reader::read_string): Update call accordingly.

From-SVN: r273510