[ADT] IntervalMap: fix setStart and setStop

Summary:
These functions currently require that the new closed interval has a length of
at least 2.  They also currently permit empty half-open intervals.  This patch
defines nonEmpty in each traits structure and uses it to correct the
implementations of setStart and setStop.

Reviewers: stoklund, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26064

llvm-svn: 285957