[flang] Allow ENTRY function result symbol usage before the ENTRY

In a function, ENTRY E without an explicit RESULT() creates a
function result entity also named E that is storage associated with
the enclosing function's result.  f18 was emitting an incorrect error
message if that function result E was referenced without any
declaration prior to its ENTRY statement when it should have been
implicitly declared instead.

Differential Revision: https://reviews.llvm.org/D125144