[libclang] Fix a trivial error introduced in D57946.

The value for CXCursor_ConvergentAttr is not 420. I'm not really sure how easy
it is to test this, and I'm not familiar with the python bindings, just noticed
the error while looking at D57946 to write D58570.

Differential Revision: https://reviews.llvm.org/D58571

llvm-svn: 354823