[flang] Lower statement function references in HLFIR

Enable lowering of statement function references in HLFIR.  This follows
the same principle as statement function lowering with the current
lowering:
- Actual arguments are lowered and mapped to the statement function
  dummy symbols.
- "HostAssociated" symbols are mapped to their host values (these are
  the symbols referred to inside the statement function expressions that
  are not statement function dummies. e.g: `x` in `stmt_func(i) =
  x(i)`).
- The statement function expression is evaluated.

evaluate::SetLength has to be lowered to deal with statement functions
returning characters since the front-end is generating one to ensure the
statement function expression value is trimmed/padded to match the statement
function declared type.

Differential Revision: https://reviews.llvm.org/D140220