python: add bindings for children of diagnostics

This exposes the Clang API bindings clang_getChildDiagnostics (which returns a
CXDiagnosticSet) and clang_getNumDiagnosticsInSet / clang_getDiagnosticInSet (to
traverse the CXDiagnosticSet), and adds a helper children property in the Python
Diagnostic wrapper.

Also, this adds the missing OVERLOAD_CANDIDATE (700) cursor type.

Patch by Hanson Wang!

llvm-svn: 268167