[FileCheck] Don't permit overlapping CHECK-DAG

[FileCheck] Don't permit overlapping CHECK-DAG

That is, make CHECK-DAG skip matches that overlap the matches of any
preceding consecutive CHECK-DAG directives.  This change makes
CHECK-DAG more consistent with other directives, and there is evidence
it makes CHECK-DAG more intuitive and less error-prone.  See the RFC
discussion starting at:

  http://lists.llvm.org/pipermail/llvm-dev/2018-May/123010.html

Moreover, this behavior enables CHECK-DAG groups for unordered,
non-unique strings or patterns.  For example, it is useful for
verifying output or logs from a parallel program, such as the OpenMP
runtime.

This patch also implements the command-line option
-allow-deprecated-dag-overlap, which reverts CHECK-DAG to the old
overlapping behavior.  This option should not be used in new tests.
It is meant only for the existing tests that are broken by this change
and that need time to update.

See the following bugzilla issue for tracking of such tests:

  https://bugs.llvm.org/show_bug.cgi?id=37532

Patches to add -allow-deprecated-dag-overlap to those tests will
follow immediately.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D47106

llvm-svn: 336830