[flang][OpenMP] Add semantic check for teams nesting

This patch implements the following check for TEAMS construct:
```
OpenMP Version 5.0 Teams construct restriction: A teams region can
only be strictly nested within the implicit parallel region or a target
region. If a teams construct is nested within a target construct, that
target construct must contain no statements, declarations or directives
outside of the teams construct.
```

Also add one test case for the check.

Reviewed By: kiranchandramohan, clementval

Differential Revision: https://reviews.llvm.org/D106335