[Flang][OpenMP] Fix semantics check for nested DISTRIBUTE (#91592)

Composite OpenMP constructs where DISTRIBUTE is the first leaf
construct, as well as standalone DISTRIBUTE constructs, are allowed
inside of TEAMS regions.

Before this patch, nesting a DISTRIBUTE construct inside of a combined
TARGET TEAMS construct was disallowed, which it shouldn't be. Now both
TEAMS and TARGET TEAMS constructs can be immediate parents of DISTRIBUTE
constructs.