[flang] If it's got an argument keyword, it can't become an array reference

Array references like A(1) are commonly misparsed as function references,
since the parser has almost no semantic context, and the parse tree is
fixed up later by semantics once it can be disambiguated.  In a case
like A(I=1), however, the presence of an argument keyword must prevent
conversion into an array reference.  (It might still also be a structure
constructor.)

Differential Revision: https://reviews.llvm.org/D142765