Create populate_isa() to de-duplicate logic to populate @ISA.

Create populate_isa() to de-duplicate logic to populate @ISA.

Previously yylex() was conditionally populating @AnyDBM_File::ISA (if it was not
set, and the token dbmopen was seen), and init_predump_symbols() was populating
@IO::File::ISA (unconditionally, but this is so early that nothing previously
could have set it). This refactoring eliminates code duplication.