[flang] Support for character array formats

A character array can be used as a format in an I/O data transfer
statement, with the interpretation that its elements are concatenated
in element order to constitute the format.

Support in the runtime with an extra optional descriptor argument
to six I/O API calls; support in semantics by removing an earlier
check for a simply contiguous array presented as a format.

Some work needs to be done in lowering to pass a character array
descriptor to the I/O runtime API when present

Differential Revision: https://reviews.llvm.org/D132167