[OpenMP][OMPT][GOMP] task frame support in KMP_API_NAME_GOMP_PARALLEL_SECTIONS

KMP_API_NAME_GOMP_PARALLEL_SECTIONS function was missing the task frame support.
This patch introduced a fix responsible to set properly the exit_frame of
the innermost implicit task that corresponds to the parallel section construct,
as well as the enter_frame of the task that encloses the mentioned implicit task.

This patch also introduced a simple test case sections_serialized.c that contains
serialized parallel section construct and validates whether the mentioned
task frames are set correctly.

Differential Revision: https://reviews.llvm.org/D112205