[Flang][OpenMP][Sema] Add semantics checks for REQUIRES directive

This patch adds semantics checks for REQUIRES directives appearing after other
directives that are affected by them. In particular, it adds checks for device
constructs appearing after device-related REQUIRES directives and for the
`atomic_default_mem_order` clause appearing after atomic operations where the
memory order is not explicitly specified.

This is patch 2/5 of a series splitting D149337 to simplify review.

Depends on D157710.

Differential Revision: https://reviews.llvm.org/D157722