ClangCodeModel: Ignore child diagnostics of type 'note' and 'ignored'

This avoids appending "previous declaration here" to the parent
diagnostic.

Clang's diagnostics are only used in the C++ editor for denoting code
warnings.

Task-number: QTCREATORBUG-13398
Change-Id: I8ed5ce2cdaa828877596745e29bb55b2d23f9735
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>