Switch all of the indirect-call code in IRGen to FunctionPointer.

To make this stick, I've disallowed direct use of that overload of
CreateCall.  I've left the Constant overloads available, but eventually
we might want to consider fixing those, too, just to get all of this
code out of the business of manually remembering to pass around
attributes and calling conventions.

The test changes reflect the fact that we weren't really setting
attributes consistently at all, in this case on value witnesses.

lib/IRGen/GenKeyPath.cpp

@@ -15,6 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "Callee.h"
 #include "ConstantBuilder.h"
 #include "Explosion.h"
 #include "GenClass.h"
@@ -72,7 +73,7 @@ getAccessorForComputedComponent(IRGenModule &IGM,
   if (requirements.empty()) {
     return accessorFn;
   }
-  
+
   auto accessorFnTy = accessorFn->getType()->getPointerElementType();
   
   // Otherwise, we need a thunk to unmarshal the generic environment from the
@@ -142,9 +143,9 @@ getAccessorForComputedComponent(IRGenModule &IGM,
                              forwardingSubs,
                              &witnessMetadata,
                              forwardedArgs);
-    auto call = IGF.Builder.CreateCall(accessorFn, forwardedArgs.claimAll());
-    if (whichAccessor == Getter)
-      call->addAttribute(1, llvm::Attribute::StructRet);
+    auto fnPtr = FunctionPointer::forDirect(IGM, accessorFn,
+                                          accessor->getLoweredFunctionType());
+    IGF.Builder.CreateCall(fnPtr, forwardedArgs.claimAll());
     
     IGF.Builder.CreateRetVoid();
   }