Do not block on explicit task depending on proxy task

Consider the following code:

    int dep;
    #pragma omp target nowait depend(out: dep)
    {
        sleep(1);
    }
    #pragma omp task depend(in: dep)
    {
        printf("Task with dependency\n");
    }
    printf("Doing some work...\n");

In its current state the runtime will block on the second task and not
continue execution.

Differential Revision: https://reviews.llvm.org/D23116

llvm-svn: 277992