[mlir] move LLVM target import header and tests

[ftynse]

Summary

Move Target/LLVMIR.h to target/LLVMIR/Import.h to better reflect the purpose of
this file. Also move all LLVM IR target tests under the LLVMIR directory.

mlir/include/mlir/Target/LLVMIR/Import.h

-//===- LLVMIR.h - MLIR to LLVM IR conversion --------------------*- C++ -*-===//
+//===- Import.h - LLVM IR To MLIR translation -------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
-// This file declares the entry point for the MLIR to LLVM IR conversion.
+// This file declares the entry point for the LLVM IR to MLIR conversion.
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef MLIR_TARGET_LLVMIR_H
-#define MLIR_TARGET_LLVMIR_H
+#ifndef MLIR_TARGET_LLVMIR_IMPORT_H
+#define MLIR_TARGET_LLVMIR_IMPORT_H
 
 #include "mlir/Support/LLVM.h"
 #include "llvm/ADT/StringRef.h"
 #include <memory>
 
 // Forward-declare LLVM classes.
 namespace llvm {
 class Module;

 /// to the error handler registered with the MLIR context, if any (obtained from
 /// the MLIR module), and return `{}`.
 OwningModuleRef
 translateLLVMIRToModule(std::unique_ptr<llvm::Module> llvmModule,
                         MLIRContext *context);
 
 } // namespace mlir
 
-#endif // MLIR_TARGET_LLVMIR_H
+#endif // MLIR_TARGET_LLVMIR_IMPORT_H

mlir/lib/Target/LLVMIR/ConvertFromLLVMIR.cpp

 //
 //===----------------------------------------------------------------------===//
 
 #include "mlir/Dialect/LLVMIR/LLVMDialect.h"
 #include "mlir/IR/Builders.h"
 #include "mlir/IR/BuiltinOps.h"
 #include "mlir/IR/BuiltinTypes.h"
 #include "mlir/IR/MLIRContext.h"
-#include "mlir/Target/LLVMIR.h"
+#include "mlir/Target/LLVMIR/Import.h"
 #include "mlir/Translation.h"
 
 #include "llvm/ADT/TypeSwitch.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/InlineAsm.h"