[OpenMP][NFC] Refactor code for interop parts of 'init' and 'append_args' clauses

The 'init' clause allows an interop-modifier of prefer_type(list) and
and interop-types 'target' and 'targetsync'.

The 'append_args' clause uses an append-op that also includes
interop-types ('target' and 'targetsync') and will allow
a prefer_type list in the next OpenMP version.

This change adds a helper struct OMPInteropInfo and uses it in the parsing
of both the 'init' and 'append_args' clauses.

One OMPInteropInfo object represents the info in a single 'init' clause.
Since 'append_args' allows a variable number of interop items it will
require an array of OMPInteropInfo objects once that is supported.

Differential Revision: https://reviews.llvm.org/D132171