[flang] Improve handling of generics with same name as type or procedure

Create symbols for generics in a pre-pass over the specification part so
it is easier to handle cases when they have the same name as a derived
type or subprogram. This is done by calling `PreSpecificationConstruct`
on each `SpecificationConstruct` of a specification part before we
continue walking it. The generics symbols are created there and the same
mechanism will be used to handle forward references to derived types.

Report an error when the same name is used for a generic interface,
derived type, and subprogram.

Improve the error message issued when a procedure and generic interface
have the same name but the procedure is not a specific of the generic.

Change `SayAlreadyDeclared` to report the error on the second occurence
of the name when possible. This can arise for declarations the are
processed out of order, e.g. contained subprograms and generic interfaces.

Avoid multiple "already declared" errors for the case when a contained
subprogram has the same name as a declared entity. We first create the
symbol with SubprogramNameDetails, then replace it with the entity (and
report the error), then replace it with the real subprogram (and get the
error again). By setting and checking the error flag we avoid the second
error.

Original-commit: flang-compiler/f18@48fc0767834154a7dbe9d5a964e603855ca88ab5
Reviewed-on: https://github.com/flang-compiler/f18/pull/614
Tree-same-pre-rewrite: false