[libclang] Add CXBinaryOperatorKind and CXUnaryOperatorKind

Adds 2 new functions to the C libclang api for retrieving operator
kinds for binary and unary operators from cursors. Also adds 2
functions for retrieving the spelling of the new enums.

Fixes https://github.com/llvm/llvm-project/issues/29138
Differential Revision: https://reviews.llvm.org/D150910