Make lto.exp work with Tcl 8.4

Make lto.exp work with Tcl 8.4

"dict" was added in Tcl 8.5, but until a couple of weeks ago the
testsuite had worked with 8.4.

This patch uses arrays instead, like we do for the caching in
target-supports.exp.  It is a bit uglier than using dicts was,
but hopefully not too bad...

2018-02-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/testsuite/
	* lib/lto.exp (lto_handle_diagnostics): Remove messages_by_file
	argument and use dg-messages-by-file instead.  Expect it to be
	an array rather than a dict.
	(lto-link-and-maybe-run): Remove messages_by_file argument and
	use an upvar for dg-messages-by-file.  Update call to
	lto_handle_diagnostics.
	(lt-get-options): Treat dg-messages-by-file as an array
	rather than a dict.
	(lto-get-options-main): Likewise.  Set the entry rather than appending.
	(lto-execute): Treat dg-messages-by-file as an array rather than
	a dict.  Update call to lto-link-and-maybe-run.

From-SVN: r257397