[flang] Include source information in an invalid file-unit-number message

An io-unit that is an internal-file-variable is syntactically identical
to a file-unit-number expression that is a variable reference.  An
ambiguous unit is initially parsed as an internal-file-variable.  If
semantic analysis determines that the unit is not of character type,
it is rewritten as an internal-file-variable.  This modification must
retain source coordinate information.

Differential revision: https://reviews.llvm.org/D91375