[IR] Export ConstantFold.h header (NFC)

[nikic]

Summary

This is in preparation for https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179. As part of that change, we'll want to invoke some of these constant folding APIs explicitly, as it won't happen as part of ConstantExpr::getXYZ() anymore.

Ideally, we'd merge these with the DL-aware constant folding APIs and only call those, but this is not easily possible for some current usages (most important IRBuilder, which uses DL-independent constant folding by default, and some major layering changes would be needed to change that).

This is basically a reboot of D115035 with different motivation.

[nhaehnle]

LGTM

[mib]

@nikic this broke the lldb macOS bot because you didn't update the module map. 148071fbae3f9a1015969376a75a47d433c49a13 should fix that.

llvm/include/llvm/IR/ConstantFold.h

-//===-- ConstantFolding.h - Internal Constant Folding Interface -*- C++ -*-===//
+//==-- ConstantFold.h - DL-independent Constant Folding Interface -*- 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 defines the (internal) constant folding interfaces for LLVM.  These
-// interfaces are used by the ConstantExpr::get* methods to automatically fold
-// constants when possible.
+// This file defines the DataLayout-independent constant folding interface.
+// When possible, the DataLayout-aware constant folding interface in
+// Analysis/ConstantFolding.h should be preferred.
+//
+// These interfaces are used by the ConstantExpr::get* methods to automatically
+// fold constants when possible.
 //
 // These operators may return a null object if they don't know how to perform
 // the specified operation on the specified constant types.
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIB_IR_CONSTANTFOLD_H
-#define LLVM_LIB_IR_CONSTANTFOLD_H
+#ifndef LLVM_IR_CONSTANTFOLD_H
+#define LLVM_IR_CONSTANTFOLD_H
 
 #include "llvm/ADT/Optional.h"
 #include "llvm/IR/InstrTypes.h"
 
 namespace llvm {
   template <typename T> class ArrayRef;
   class Value;
   class Constant;
   class Type;
 
   // Constant fold various types of instruction...
   Constant *ConstantFoldCastInstruction(
     unsigned opcode,     ///< The opcode of the cast
     Constant *V,         ///< The source constant

llvm/lib/IR/ConstantFold.cpp

 // ConstantExpr::get* methods to automatically fold constants when possible.
 //
 // The current constant folding implementation is implemented in two pieces: the
 // pieces that don't need DataLayout, and the pieces that do. This is to avoid
 // a dependence in IR on Target.
 //
 //===----------------------------------------------------------------------===//
 
-#include "ConstantFold.h"
+#include "llvm/IR/ConstantFold.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/GlobalAlias.h"
 #include "llvm/IR/GlobalVariable.h"

llvm/lib/IR/Constants.cpp

 //===-- Constants.cpp - Implement Constant nodes --------------------------===//
 //
 // 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 implements the Constant* classes.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/Constants.h"
-#include "ConstantFold.h"
 #include "LLVMContextImpl.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/ConstantFold.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/GlobalAlias.h"
 #include "llvm/IR/GlobalIFunc.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Instructions.h"