[flang][NFC] Add accessors to equivalence and common blocks

Add a way to get mutable equivalence sets to Scope so that they can
have sizes and offsets assigned to them.

Change CommonBlockDetails to have mutable symbols so that they can have
sizes and offets assigned to them. This also allows the removal of some
`const_cast`s.

Add MutableSymbolRef and MutableSymbolVector as mutable analogs to
SymbolRef and SymbolVector. Replace uses of equivalent types with those
names.

Differential Revision: https://reviews.llvm.org/D79346