… as records

This ensures the PL/pgSQL lexer detects variable references to such
variables as variables, instead of erroring out with
"[var.name] is not a known variable".

The trade-off here is that we might incorrectly detect custom data types
registered outside of pg_catalog as being records, instead of data types.

…references

This makes sure function bodies that contain ALIAS FOR statements to
rename positional references like $1 get parsed correctly, instead of
erroring out with "variable "$1" does not exist".

…param_varno

This ensures functions with statements like "RETURN NEXT;" or "RETURN;"
don't error out.

…sitive compare

This requires the data type parsing to correctly set the collation field,
which for now we do when the type is defined as "text".

In passing we also adjust the hardcoded collection in get_collation_oid
to use DEFAULT_COLLATION_OID (100), which seems better than using -1.