[flang] Add hlfir.as_expr definition

hlfir.as_expr allows taking a value from a character, derived type,
or array expressions. This will allow implementing parentheses.

Combining as_expr + hlfir.associate will allow creating a variable copy
into a new temporary variable.

A later patch will add the ability to "move" a variable into an
expression (to give ownership of the variable storage to the expression,
with the commitment that the variable will not be used anymore).

Differential Revision: https://reviews.llvm.org/D139519