Fix honoring of OMP_THREAD_LIMIT in the teams construct

Fix for crash in the teams construct in case user sets OMP_THREAD_LIMIT to a
number less than the number of processors. Now the number of threads will be
silently reduced if the user didn't specify teams parameters or with a
warning if the user specified teams parameters conflicting with
OMP_THREAD_LIMIT.

Differential Revision: http://reviews.llvm.org/D14732

llvm-svn: 254322