[ArgPromotion] Move ArgPart and OffsetAndArgPart to anonymous namespace

[psamolysov]

Summary

The structure ArgPart and alias OffsetAndArgPart have been moved
into the anonymous namespace. NFC.

[psamolysov]

@aeubanks Thank you for the review and landing.

llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

 using namespace llvm;
 
 #define DEBUG_TYPE "argpromotion"
 
 STATISTIC(NumArgumentsPromoted, "Number of pointer arguments promoted");
 STATISTIC(NumByValArgsPromoted, "Number of byval arguments promoted");
 STATISTIC(NumArgumentsDead, "Number of dead pointer args eliminated");
 
+namespace {
+
 struct ArgPart {
   Type *Ty;
   Align Alignment;
   /// A representative guaranteed-executed load instruction for use by
   /// metadata transfer.
   LoadInst *MustExecLoad;
 };
+
 using OffsetAndArgPart = std::pair<int64_t, ArgPart>;
 
+} // end anonymous namespace
+
 static Value *createByteGEP(IRBuilderBase &IRB, const DataLayout &DL,
                             Value *Ptr, Type *ResElemTy, int64_t Offset) {
   // For non-opaque pointers, try to create a "nice" GEP if possible, otherwise
   // fall back to an i8 GEP to a specific offset.
   unsigned AddrSpace = Ptr->getType()->getPointerAddressSpace();
   APInt OrigOffset(DL.getIndexTypeSizeInBits(Ptr->getType()), Offset);
   if (!Ptr->getType()->isOpaquePointerTy()) {
     Type *OrigElemTy = Ptr->getType()->getNonOpaquePointerElementType();