Remove the calls to PerformPendingInstantiations prematurely.

[vgvassilev]

While differentiating we are calling Sema::PerformPendingInstantiations which eagerly instantiates pending templates. We do this because we want to make sure if we differentiate a templated function it has a body. However, that may lead to recursive parsing on Windows where the template instantiation can be connected to additional required parsing. In turn that is problematic for Clang because it expects the state of the compiler to be at the global scope.