[flang] (NFC) Expose internal idiom as utility API

Add overloads to AsGenericExpr() in Evaluate/tools.h to take care
of wrapping an untyped DataRef or bare Symbol in a typed Designator
wrapped up in a generic Expr<SomeType>.  Use the new overloads to
replace a few instances of code that was calling TypedWrapper<>()
with a dynamic type.

This new tool will be useful in lowering to drive some code that
works with typed expressions (viz., list-directed I/O list items)
when starting with only a bare Symbol (viz., NAMELIST).

Differential Revision: https://reviews.llvm.org/D102352