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

[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).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330557 91177308-0d34-0410-b5e6-96231b3b80d8