[Flang][OpenMP] Improve support for `if` clause on combined constructs

This patch adds support for matching multiple OpenMP `if` clauses to their
specified directive in a combined construct. It also enables this clause to be
attached by name to `simd` and `teams` directives, in addition to the others
that were already supported.

This patch on its own cannot yet be tested because there is currently no
lowering to MLIR support for any combined construct containing two or more
OpenMP directives that can have an `if` clause attached.

Depends on D155981.

Differential Revision: https://reviews.llvm.org/D156313