[flang] Extension: don't require commas between most edit descriptors in formats

The standard explicitly allows a comma to be omitted between a 'P'
edit descriptor and a following numeric edit descriptor (e.g., 1PE10.1),
and before and after a '/' edit descriptor, but otherwise requires them
between edit descriptors.  Most implementations, however, only require
commas where they prevent ambiguity, and accept things like 1XI10.
This extension is already assumed by the static FORMAT checker in
semantics.  Patch the runtime to behave accordingly.

Differential Revision: https://reviews.llvm.org/D120747