[flang] Resolve names in ASSOCIATE and SELECT TYPE

Create `AssocEntityDetails` for symbols that represent entities
identified by the associate-name in ASSOCIATE and SELECT TYPE
constructs.

For ASSOCIATE, create a new scope for the associated entity.
For SELECT TYPE, create a new scope for each of type guard blocks.
Each one contains an associated entity with the appropriate type.

For SELECT TYPE, also create a place-holder symbol for the
associate-name in the SELECT TYPE statement. The real symbols
are in the new scopes and none of them is uniquely identified
with the associate-name.

Handling of `Selector` is common between these, with
`associate-name => expr | variable` recorded in
`ConstructVisitor::association_`.

When the selector is an expression, derive the type of the associated
entity from the type of the expression. This required some refactoring
of how `DeclTypeSpec`s are created. The `DerivedTypeSpec` that comes
from and expression is const so we can only create const `DeclTypeSpec`s
from it. But there were times during name resolution when we needed to
set type parameters in the current `DeclTypeSpec`. Now the non-const
`DerivedTypeSpec` is saved separately from the const `DeclTypeSpec`
while we are processing a declaration type spec. This makes it
unnecessary to save the derived type name.

Add a type alias for `common::Indirection` to reduce verbosity.

Original-commit: flang-compiler/f18@b7668cebe49a122ea23c89c81eafdeba243bbfaf
Reviewed-on: https://github.com/flang-compiler/f18/pull/261
Tree-same-pre-rewrite: false