EDSC: unify Expr storage

EDSC expressions evolved to have different types of underlying storage.
Separate classes are used for unary, binary, ternary and variadic expressions.
The latter covers all the needs of the three special cases.  Remove these
special cases and use a single ExprStorage class everywhere while maintaining
the same APIs at the Expr level (ExprStorage is an internal implementation
class).

This is step 1/n to converging EDSC expressions and Ops and making EDSCs
support custom operations.

PiperOrigin-RevId: 233704912