[OpenMP] Update CHECK-DAG usage in for_codegen.cpp

This patch adjusts a test not to depend on deprecated FileCheck
behavior that permits overlapping matches within a block of CHECK-DAG
directives.  Thus, this patch also removes uses of FileCheck's
-allow-deprecated-dag-overlap command-line option.

Specifically, the FileCheck variables DBG_LOC_START, DBG_LOC_END, and
DBG_LOC_CANCEL were all set to the same value.  As a result, three
TERM_DEBUG-DAG patterns, one for each variable, all matched the same
text under the old overlapping behavior.  Under the new
non-overlapping behavior, that's not permitted.  This patch's solution
is to replace these variables with one variable and replace these
patterns with one pattern.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D54764

llvm-svn: 347350