Introduce GOMP_taskloop API

This patch introduces GOMP_taskloop to our API. It adds GOMP_4.5 to our
version symbols. Being a wrapper around __kmpc_taskloop, the function
creates a task with the loop bounds properly nested in the shareds so that
the GOMP task thunk will work properly. Also, the firstprivate copy constructors
are properly handled using the __kmp_gomp_task_dup() auxiliary function.

Currently, only linear spawning of tasks is supported
for the GOMP_taskloop interface.

Differential Revision: https://reviews.llvm.org/D45327

llvm-svn: 330282