[flang] Do not propagate BIND(C) from main entry to ENTRY (#67554)

15.6.2.6 point 11/12/13 tells that entries do inherit the
RECURSIVE/PURE/ELEMENTAL aspects from the main entry, but nothing as
such is said for BIND(C). It seems each entry can independently have
BIND(C) or not.

Update characterization to not propagate this info in-between entries.

Add a lowering test for a tricky case of the character return where the
return ABI is different for the result with and without BIND(C), but the
results storage should be associated regardless of the ABI.