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/unittests/Analysis/OrderedInstructionsTest.cpp b/llvm/unittests/Analysis/OrderedInstructionsTest.cpp
index 58eb0e5..1f4002f 100644
--- a/llvm/unittests/Analysis/OrderedInstructionsTest.cpp
+++ b/llvm/unittests/Analysis/OrderedInstructionsTest.cpp
@@ -25,7 +25,7 @@
   IRBuilder<> B(Ctx);
   FunctionType *FTy =
       FunctionType::get(Type::getVoidTy(Ctx), {B.getInt8PtrTy()}, false);
-  Function *F = Function::Create(FTy, Function::ExternalLinkage, "f", M);
+  Function *F = cast<Function>(M.getOrInsertFunction("f", FTy));
 
   // Create the function as follow and check for dominance relation.
   //