[flang] Do not die on typeless source expression in allocate.

While fixing 594, it appears the CHECK in allocate for non null
expression type pointer was too harsh as it could be a user error.
e.g: a boz used as source.

Original-commit: flang-compiler/f18@cbacdeaa0eb8077810db85d5b4469f797d524900
Reviewed-on: https://github.com/flang-compiler/f18/pull/607
Tree-same-pre-rewrite: false