[flang] Set default form if unspecified at end of OPEN statement

The standard specifies that the form must be set to a default at the
end of an open statement (C.7.4 point 2) where it was not specified.
Only in the case of a pre-connected unit can the form be deferred
until the first data transfer statement.

Set back the default form setting in OpenStatementState::EndIoStatement
that had been removed when deferring the pre-connected unit from setting
in 199a623e.

Also return "UNDEFINED" instead of "UNKNOWN" to the FORM inquiry when the
form was not yet specified as defined in 12.10.2.12.

Related test: syntax 215a216.

Differential Revision: https://reviews.llvm.org/D108990