withUnsafeMutablePointer(to: &context) { (context) -> () in
      Builtin.onceWithContext(oncePtr._rawValue, _createStringTableCache // @convention(swift) -> @convention(c) conversion
                              , context._rawValue)
    }

// someFunc's source is available
takesCPtr(someFunc) // okay
let f = someFunc; takesCPtr(f) // error
let g : @convention(c) ... = someFunc; takesCPtr(g) // okay

    if (srcRepTy->getRepresentation() !=
        FunctionTypeRepresentation::CFunctionPointer) {
      // A "conversion" of a DeclRef a C function pointer is done by referencing
      // the thunk (or original C function) with the C calling convention.
      result = emitCFunctionPointer(SGF, e);

// emitCFunctionPointer
    SGF.SGM.diagnose(expr->getLoc(),
                     diag::c_function_pointer_from_function_with_context,
                     /*closure*/ constant.hasClosureExpr(),
                     kind);