[fir] Add !fir.field type conversion

[clementval]

Summary

This patch adds conversion for !fir.field types.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Eric Schweitz <[e-mail]>

[kiranchandramohan]

LGTM.

flang/lib/Optimizer/CodeGen/TypeConverter.h

LLVMTypeConverter(mlir::ModuleOp module)
     addConversion([&](fir::LogicalType boolTy) {
       return mlir::IntegerType::get(
           &getContext(), kindMapping.getLogicalBitsize(boolTy.getFKind()));
     });
     addConversion(
         [&](fir::PointerType pointer) { return convertPointerLike(pointer); });
     addConversion(
         [&](fir::RecordType derived) { return convertRecordType(derived); });
+    addConversion([&](fir::FieldType field) {
+      // Convert to i32 because of LLVM GEP indexing restriction.
+      return mlir::IntegerType::get(field.getContext(), 32);
+    });
     addConversion(
         [&](fir::ComplexType cmplx) { return convertComplexType(cmplx); });
     addConversion(
         [&](fir::RealType real) { return convertRealType(real.getFKind()); });
     addConversion(
         [&](fir::ReferenceType ref) { return convertPointerLike(ref); });
     addConversion([&](fir::SequenceType sequence) {
       return convertSequenceType(sequence);

flang/test/Fir/types-to-llvm.fir

 
 func private @foo0(%arg0: !fir.boxchar<1>)
 // CHECK-LABEL: foo0
 // CHECK-SAME: !llvm.struct<(ptr<i8>, i64)>
 
 func private @foo1(%arg0: !fir.boxchar<2>)
 // CHECK-LABEL: foo1
 // CHECK-SAME: !llvm.struct<(ptr<i16>, i64)>
+
+// -----
+
+// Test `!fir.field` conversion
+
+func private @foo0(%arg0: !fir.field)
+// CHECK-LABEL: foo0
+// CHECK-SAME: i32