[python bindings] Fix Cursor.result_type for ObjC method declarations - Bug 36677

Summary:
In cindex.py, Cursor.result_type called into the wrong libclang
function, causing cursors for ObjC method declarations to return invalid
result types. Fixes Bug 36677.

Reviewers: jbcoe, rsmith

Reviewed By: jbcoe

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D45671

Patch by kjteske (Kyle Teske).

llvm-svn: 330557