Enable debugger support

These changes enable external debuggers to conveniently interface with 
the LLVM OpenMP Library.  Structures are added which describe the important
internal structures of the OpenMP Library e.g., teams, threads, etc.
This feature is turned on by default (CMake variable LIBOMP_USE_DEBUGGER)
and can be turned off with -DLIBOMP_USE_DEBUGGER=off.

Differential Revision: http://reviews.llvm.org/D10038

llvm-svn: 241832