tree-cfg: Fix call to next_discriminator_for_locus()

While testing future 64-bit location_t support, I ran into an
-fcompare-debug issue that was traced back here. Despite the name,
next_discriminator_for_locus() is meant to take an integer line number
argument, not a location_t. There is one call site which has been passing a
location_t instead. For the most part that is harmless, although in case
there are two CALL stmts on the same line with different location_t, it may
fail to generate a unique discriminator where it should. If/when location_t
changes to be 64-bit, however, it will produce an -fcompare-debug
failure. Fix it by passing the line number rather than the location_t.

I am not aware of a testcase that demonstrates any observable wrong
behavior, but the file debug/pr53466.C is an example where the discriminator
assignment is indeed different before and after this change.

gcc/ChangeLog:

	* tree-cfg.cc (assign_discriminators): Fix incorrect value passed to
	next_discriminator_for_locus().