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/AtomicExpandPass.cpp b/llvm/lib/CodeGen/AtomicExpandPass.cpp
index 6e8d68e..2d91594 100644
--- a/llvm/lib/CodeGen/AtomicExpandPass.cpp
+++ b/llvm/lib/CodeGen/AtomicExpandPass.cpp
@@ -1754,7 +1754,7 @@
   for (Value *Arg : Args)
     ArgTys.push_back(Arg->getType());
   FunctionType *FnType = FunctionType::get(ResultTy, ArgTys, false);
-  FunctionCallee LibcallFn =
+  Constant *LibcallFn =
       M->getOrInsertFunction(TLI->getLibcallName(RTLibType), FnType, Attr);
   CallInst *Call = Builder.CreateCall(LibcallFn, Args);
   Call->setAttributes(Attr);