[flang] Change BeginAttrs to return true like BeginDecl

This is just for convenience. Any Pre function that calls BeginAttrs
must return true.

Original-commit: flang-compiler/f18@5a7bc11ba43f0e0d0dbdfe845f06c86274fde8ad
Reviewed-on: https://github.com/flang-compiler/f18/pull/173
Tree-same-pre-rewrite: false