cindex.py: Avoid deprecated function

Implement Diagnostic::category_name() using clang_getDiagnosticCategoryText()
instead of the deprected clang_getDiagnosticCategoryName().

Preserves existing behaviour and API covered by existing tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206712 91177308-0d34-0410-b5e6-96231b3b80d8