[Flang][OpenMP] Lowering of host-evaluated clauses

This patch adds support for lowering OpenMP clauses and expressions attached to
constructs nested inside of a target region that need to be evaluated in the
host device. This is done through the use of the `OpenMP_HostEvalClause`
`omp.target` set of operands and entry block arguments.

When lowering clauses for a target construct, a more involved
`processHostEvalClauses()` function is called, which looks at the current and
potentially other nested constructs in order to find and lower clauses that
need to be processed outside of the `omp.target` operation under construction.
This populates an instance of a global structure with the resulting MLIR
values.

The resulting list of host-evaluated values is used to initialize the
`host_eval` operands when constructing the `omp.target` operation, and then
replaced with the corresponding block arguments after creating that operation's
region.

Afterwards, while lowering nested operations, those that might potentially be
evaluated in the host (e.g. `num_teams`, `thread_limit`, `num_threads` and
`collapse`) check first whether there is an active global host-evaluated
information structure and whether it holds values referring to these clauses.
If that is the case, the stored values (referring to `omp.target` entry block
arguments at that stage) are used instead of lowering clauses again.