multiline.exp: complain about mismatched dg-{begin|end}-multiline-output

multiline.exp: complain about mismatched dg-{begin|end}-multiline-output

Mismatched dg-{begin|end}-multiline-output directives are currently
silently ignored, leading to difficult-to-diagnose test failures
involving excess output.

This patch makes multiline.exp complain about them.

gcc/testsuite/ChangeLog:
	* lib/multiline.exp (dg-begin-multiline-output): Issue an error if
	there hasn't been a dg-end-multiline-output since the last
	dg-begin-multiline-output.
	(dg-end-multiline-output): Issue an error if there hasn't been a
	dg-begin-multiline-output.  Reset _multiline_last_beginning_line
	as soon possible.  Rename "line" to "last_line".

From-SVN: r265046