[OMPT] Initialize task fields only if needed

Because __kmp_task_init_ompt is called for every initial task in each thread
and always generated task ids, this was a big performance issue on bigger
systems even without any tool attached.  After changing the initialization 
interface to ompt_tool, we can now rely on already knowing whether a tool is
attached and OMPT is enabled at this point.

Patch by Jonas Hahnfeld

Differential Revision: http://reviews.llvm.org/D13494

llvm-svn: 249855