Don't use already-used DUChain identity

Please check/update the following page before introducing new DUChain
identities:

https://techbase.kde.org/KDevelop5/DUChainIdentities

This fixes a crash on start when both kdev-python & kdev-php were
loaded:

ASSERT: "!m_factories.contains(identity)" in file

/home/kfunk/devel/src/kf5/kdevelop/kdevplatform/language/duchain/types/typeregister.cpp,
line 86