fix: address symm and label bugs from #3408 (#3429)

#3408 was somewhat large and didn't properly test the symm and label
attribute code after edits to the builtin versions.

This migrates the code for generating labeled attributes from Init back
to Lean so that the required definitions are in scope.

This also addresses a mistake in the symm elaborator that prevented symm
without location information from elaborating.

Both fixes have been tested on the Std test suite and successfully
passed.