[Frontend] Add leaf constructs and association to OpenMP/ACC directives (#83625)

Add members "leafConstructs" and "association" to .td describing
OpenMP/ACC directives. The naming follows the terminology used in the
OpenMP standard: a "leaf" construct is a construct that is itself not a
composition or a combination of other constructs, and "association" is
the source language construct to which the directive applies (e.g. loop,
block, etc.)

The tblgen-generated output then contains two additional functions
- getLeafConstructs(D), and
- getDirectiveAssociation(D)
plus "enum class Association", all in namespaces "llvm::omp" and
"llvm::acc".

Note: getLeafConstructs returns an empty sequence for a construct that
is itself a leaf construct.

Use the new functions to simplify a few OpenMP-related functions in
clang.