[OpenMP] Fix tasking + parallel bug

From the bug report, the runtime needs to initialize the nproc variables
(inside middle init) for each root when the task is encountered, otherwise,
a segfault can occur.

Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=36720

Differential Revision: https://reviews.llvm.org/D49996

llvm-svn: 338313