Remove redundant FunctionDecl argument from a couple functions.

This argument was added in r254554 in order to support the
pass_object_size attribute. However, in r296076, the attribute's
presence is now also represented in FunctionProtoType's
ExtParameterInfo, and thus it's unnecessary to pass along a separate
FunctionDecl.

The functions modified are:
 RequiredArgs::forPrototype{,Plus}, and
 CodeGenTypes::ConvertFunctionType.

After this, it's also (again) unnecessary to have a separate
ConvertFunctionType function ConvertType, so convert callers back to
the latter, leaving the former as an internal helper function.

llvm-svn: 352946