[clang][Builtins] Parse clang extended vectors types. (#83584)

Clang extended vector types are mangled as follows:

    '_ExtVector<' <lanes> ',' <scalar type> '>'

This is used to defetmine the builtins signature for builtins that
use parameters defined as

    typedef <scalar type> ext_vector_type_<lanes>_<scalar type> __attribute__((ext_vector_type(<lanes>)))

or 

    template <unsigned N, class T>
    using _ExtVector __attribute__((ext_vector_type(N))) = T;

For example:

    typedef double ext_vector_type_4_double __attribute__((ext_vector_type(4)))