[flang] Save and fetch analyzed Expr in Variable

Add typedExpr data member to Variable like that in Expr.
When expression analysis analyzed a Variable it stores the
resulting evaluate::Expr there.

Add GetExpr overloads in semantics/tools.h for using in statement
semantics. It gets an evaluate::Expr from an Expr, Variable, or
wrapper around one of those. It returns a const pointer so that
clients cannot modify the cached expression (and copies do not
have to be made).

Change CoarrayChecker to make use of GetExpr. It will eventually
replace all references to typedExpr in statement semantics.

Original-commit: flang-compiler/f18@b02a41efe12032d67cbc99abe43adfb4abb422bb
Reviewed-on: https://github.com/flang-compiler/f18/pull/422
Tree-same-pre-rewrite: false