[flang] Extension: Accept Hollerith actual arguments as if they were BOZ

When a Hollerith (or short character) literal is presented as an actual
argument that corresponds to a dummy argument for which a BOZ literal
would be acceptable, treat the Hollerith as if it had been a BOZ
literal in the same way -- and with the same code -- as f18 already
does for the similar extension in DATA statements.

Differential Revision: https://reviews.llvm.org/D126144