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/MIRParser/MIRParser.cpp b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
index 2fc53d7..b7e2957 100644
--- a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
+++ b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -270,9 +270,8 @@
 /// Create an empty function with the given name.
 static Function *createDummyFunction(StringRef Name, Module &M) {
   auto &Context = M.getContext();
-  Function *F =
-      Function::Create(FunctionType::get(Type::getVoidTy(Context), false),
-                       Function::ExternalLinkage, Name, M);
+  Function *F = cast<Function>(M.getOrInsertFunction(
+      Name, FunctionType::get(Type::getVoidTy(Context), false)));
   BasicBlock *BB = BasicBlock::Create(Context, "entry", F);
   new UnreachableInst(Context, BB);
   return F;