[MLIR][OpenMP] Update loop operations to use omp.loopnest

This patch updates the definition of all OpenMP loop operations
(`omp.distribute`, `omp.parallel`, `omp.wsloop`, `omp.simdloop`,
`omp.taskloop`) to expect a nested `omp.loopnest` and act as wrapper operations
instead.

Loop information is removed from these operations and their list of traits is
updated to include a new common `LoopWrapperInterface`, as well as introducing
`SingleBlockImplicitTerminator` traits to all but `omp.parallel`, which can be
a wrapper in composite constructs but also define a regular code block.

These changes make it so wrapper operations can only have a single region and
block, and that it must only have a single compatible wrapper operation or
canonical loop in addition to a terminator.

Follow-up patches will update PFT to MLIR lowering to generate MLIR conforming
to these new definitions for loops.

mlir/include/mlir/Dialect/OpenMP/OpenMPInterfaces.h

@@ -21,6 +21,9 @@
 #include "mlir/Interfaces/ControlFlowInterfaces.h"
 #include "mlir/Interfaces/SideEffectInterfaces.h"
 
+#define GET_OP_FWD_DEFINES
+#include "mlir/Dialect/OpenMP/OpenMPOps.h.inc"
+
 #include "mlir/Dialect/OpenMP/OpenMPOpsInterfaces.h.inc"
 
 namespace mlir::omp {