[clang][CodeGen] GetDefaultAlignTempAlloca uses preferred alignment

[wingo]

Summary

This function was defaulting to use the ABI alignment for the LLVM
type. Here we change to use the preferred alignment. This will allow
unification with GetTempAlloca, which if alignment isn't specified, uses
the preferred alignment.

[wingo]

I tested by building with all targets enabled and there was no change in test results, running clang, llvm, and lld tests.

[rjmccall]

Sure, I guess there's no harm in this.

clang/lib/CodeGen/CGExpr.cpp

 
 /// CreateDefaultAlignTempAlloca - This creates an alloca with the
 /// default alignment of the corresponding LLVM type, which is *not*
 /// guaranteed to be related in any way to the expected alignment of
 /// an AST type that might have been lowered to Ty.
 Address CodeGenFunction::CreateDefaultAlignTempAlloca(llvm::Type *Ty,
                                                       const Twine &Name) {
   CharUnits Align =
-    CharUnits::fromQuantity(CGM.getDataLayout().getABITypeAlignment(Ty));
+      CharUnits::fromQuantity(CGM.getDataLayout().getPrefTypeAlignment(Ty));
   return CreateTempAlloca(Ty, Align, Name);
 }
 
 void CodeGenFunction::InitTempAlloca(Address Var, llvm::Value *Init) {
   auto *Alloca = Var.getPointer();
   assert(isa<llvm::AllocaInst>(Alloca) ||
          (isa<llvm::AddrSpaceCastInst>(Alloca) &&
           isa<llvm::AllocaInst>(