[Flang][LoongArch] Add support for complex16 params/returns. (llvm#114732)

In LoongArch64, the passing and returning of type `complex16` is similar
to that of structure type like `struct {fp128, fp128}`, meaning they are
passed and returned by reference. This behavior is similar to clang, so
it can implement conveniently `iso_c_binding`.

Additionally, this patch fixes the failure in flang test
Integration/debug-complex-1.f90:
```
llvm-project/flang/lib/Optimizer/codeGen/Target.cpp:56:
not yet implemented: complex for this precision for return type

Author: ylzsx

flang/lib/Optimizer/CodeGen/Target.cpp

@@ -1091,6 +1091,13 @@ struct TargetLoongArch64 : public GenericTarget<TargetLoongArch64> {
       // Two distinct element type arguments (re, im)
       marshal.emplace_back(eleTy, AT{});
       marshal.emplace_back(eleTy, AT{});
+    } else if (sem == &llvm::APFloat::IEEEquad()) {
+      // Use a type that will be translated into LLVM as:
+      // { fp128, fp128 }   struct of 2 fp128, byval
+      marshal.emplace_back(
+          fir::ReferenceType::get(mlir::TupleType::get(
+              eleTy.getContext(), mlir::TypeRange{eleTy, eleTy})),
+          AT{/*align=*/16, /*byval=*/true});
     } else {
       typeTodo(sem, loc, "argument");
     }
@@ -1108,6 +1115,13 @@ struct TargetLoongArch64 : public GenericTarget<TargetLoongArch64> {
       marshal.emplace_back(mlir::TupleType::get(eleTy.getContext(),
                                                 mlir::TypeRange{eleTy, eleTy}),
                            AT{/*alignment=*/0, /*byval=*/true});
+    } else if (sem == &llvm::APFloat::IEEEquad()) {
+      // Use a type that will be translated into LLVM as:
+      // { fp128, fp128 }   struct of 2 fp128, sret, align 16
+      marshal.emplace_back(
+          fir::ReferenceType::get(mlir::TupleType::get(
+              eleTy.getContext(), mlir::TypeRange{eleTy, eleTy})),
+          AT{/*align=*/16, /*byval=*/false, /*sret=*/true});
     } else {
       typeTodo(sem, loc, "return");
     }

flang/test/Fir/target-rewrite-complex16.fir

@@ -1,4 +1,5 @@
 // RUN: fir-opt --target-rewrite="target=x86_64-unknown-linux-gnu" %s | FileCheck %s
+// RUN: fir-opt --target-rewrite="target=loongarch64-unknown-linux-gnu" %s | FileCheck %s
 
 // Test that we rewrite the signature and body of a func.function that returns a
 // complex<16>.