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/Target/AMDGPU/AMDGPULibFunc.h b/llvm/lib/Target/AMDGPU/AMDGPULibFunc.h
index 2354ed7..972ccb4 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPULibFunc.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPULibFunc.h
@@ -393,8 +393,8 @@
   }
   static Function *getFunction(llvm::Module *M, const AMDGPULibFunc &fInfo);
 
-  static FunctionCallee getOrInsertFunction(llvm::Module *M,
-                                            const AMDGPULibFunc &fInfo);
+  static Function *getOrInsertFunction(llvm::Module *M,
+                                       const AMDGPULibFunc &fInfo);
   static bool parse(StringRef MangledName, AMDGPULibFunc &Ptr);
 
 private: