[OPENMP]Fix PR37671: Privatize local(private) variables in untied tasks.

Summary:
In untied tasks, need to allocate the space for local variales, declared
in task region, when the memory for task data is allocated. THe function
can be interrupted and we can exit from the function in untied task
switch. Need to keep the state of the local variables in this case.
Also, the compiler should not call cleanup when exiting in untied task
switch until the real exit out of the declaration scope is met during
 execution.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, cfe-commits, sstefan1, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84457