[flang] Accept TYPE(intrinsic type) in declarations only for non-extension type

To avoid clashing with names of user derived types, the redundant
syntax TYPE(intrinsic type spec) must be interpreted as a monomorphic
derived type when "intrinsic type spec" is a single word.  This
affects TYPE(BYTE) and TYPE(DOUBLECOMPLEX), but not TYPE(DOUBLE COMPLEX)
in free form source.

Differential Revision: https://reviews.llvm.org/D123724