[swig] Don't mess with swig internals.

As of SWIG 4.0, __swig_getmethods__ and __swig_setmethods__ are no
longer defined. It appears that there's no need to mess with these
internals, we can simplify define the corresponding properties inline.

Originally I wanted to use the swig extension %attribute and
%attributeref to define properties. However, I couldn't find a way to
add documentation to these attributes. Since we already had the
properties defined inline, we might as well keep them.

Differential revision: https://reviews.llvm.org/D63530

llvm-svn: 364974