[flang] Expunge needless semantics::ProcInterface

The ProcInterface structure is used only by ProcEntityDetails; it represents
what a program might have put in parentheses in a procedure-declaration-stmt,
either the name of a procedure interface or a declaration-type-spec.

If a procedure entity has an implicit interface, the function result
type (if any) can be kept in EntityDetails::type_, which already exists
and is currently redundant for ProcEntityDetails symbols.

All that is really needed is a nullable Symbol pointer in ProcEntityDetails
to point to the procedure's explicit interface, when it has one.

Also, catch the case where a procedure has an explicit interface
and a program attempts to also give it a type.

Differential Revision: https://reviews.llvm.org/D140134