[Flang][OpenMP][Lower] Split MLIR codegen for clauses and constructs (#86963)

This patch performs several cleanups with the main purpose of
normalizing the code patterns used to trigger codegen for MLIR OpenMP
operations and making the processing of clauses and constructs
independent. The following changes are made:

- Clean up unused `directive` argument to
`ClauseProcessor::processMap()`.
- Move general helper functions in OpenMP.cpp to the appropriate section
of the file.
- Create `gen<OpName>Clauses()` functions containing the clause
processing code specific for the associated OpenMP construct.
- Update `gen<OpName>Op()` functions to call the corresponding
`gen<OpName>Clauses()` function.
- Sort calls to `ClauseProcessor::process<ClauseName>()` alphabetically,
to avoid inadvertently relying on some arbitrary order. Update some
tests that broke due to the order change.
- Normalize `genOMP()` functions so they all delegate the generation of
MLIR to `gen<OpName>Op()` functions following the same pattern.
- Only process `nowait` clause on `TARGET` constructs if not compiling
for the target device.

A later patch can move the calls to `gen<OpName>Clauses()` out of
`gen<OpName>Op()` functions and passing completed clause structures
instead, in preparation to supporting composite constructs. That will
make it possible to reuse clause processing for a given leaf construct
when appearing alone or in a combined or composite construct, while
controlling where the associated code is produced.

Author: skatrak

flang/lib/Lower/OpenMP/ClauseProcessor.cpp

@@ -832,8 +832,8 @@ createMapInfoOp(fir::FirOpBuilder &builder, mlir::Location loc,
 }
 
 bool ClauseProcessor::processMap(
-    mlir::Location currentLocation, const llvm::omp::Directive &directive,
-    Fortran::lower::StatementContext &stmtCtx, mlir::omp::MapClauseOps &result,
+    mlir::Location currentLocation, Fortran::lower::StatementContext &stmtCtx,
+    mlir::omp::MapClauseOps &result,
     llvm::SmallVectorImpl<const Fortran::semantics::Symbol *> *mapSyms,
     llvm::SmallVectorImpl<mlir::Location> *mapSymLocs,
     llvm::SmallVectorImpl<mlir::Type> *mapSymTypes) const {

flang/lib/Lower/OpenMP/ClauseProcessor.h

@@ -114,8 +114,7 @@ class ClauseProcessor {
   // They may be used later on to create the block_arguments for some of the
   // target directives that require it.
   bool processMap(
-      mlir::Location currentLocation, const llvm::omp::Directive &directive,
-      Fortran::lower::StatementContext &stmtCtx,
+      mlir::Location currentLocation, Fortran::lower::StatementContext &stmtCtx,
       mlir::omp::MapClauseOps &result,
       llvm::SmallVectorImpl<const Fortran::semantics::Symbol *> *mapSyms =
           nullptr,