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!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268167 91177308-0d34-0410-b5e6-96231b3b80d8