Revert "[opaque pointer types] Add a FunctionCallee wrapper type, and use it."

This reverts commit f47d6b38c7a61d50db4566b02719de05492dcef1 (r352791).

Seems to run into compilation failures with GCC (but not clang, where
I tested it). Reverting while I investigate.

llvm-svn: 352800
diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 01b6284..1fb67bf 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -1587,8 +1587,8 @@
   // thread's unsafe stack pointer.
   Module *M = IRB.GetInsertBlock()->getParent()->getParent();
   Type *StackPtrTy = Type::getInt8PtrTy(M->getContext());
-  FunctionCallee Fn = M->getOrInsertFunction("__safestack_pointer_address",
-                                             StackPtrTy->getPointerTo(0));
+  Value *Fn = M->getOrInsertFunction("__safestack_pointer_address",
+                                     StackPtrTy->getPointerTo(0));
   return IRB.CreateCall(Fn);
 }