[flang] Fix symbol table bugs with ENTRY statements

Dummy arguments of ENTRY statements in execution parts were
not being created as objects, nor were they being implicitly
typed.

When the symbol corresponding to an alternate ENTRY point
already exists (by that name) due to having been referenced
in an earlier call, name resolution used to delete the extant
symbol.  This isn't the right thing to do -- the extant
symbol will be pointed to by parser::Name nodes in the parse
tree while no longer being part of any Scope.

Differential Review: https://reviews.llvm.org/D102948