[flang] Move HasError, SetError to SemanticsContext

Move HasError and SetError into SemanticsContext so that we can do
consistency checks: if a symbol has an error or we want to set the
error flag, check that an error has been reported.

Original-commit: flang-compiler/f18@4f6e3a04ca4c4d46a55900551ff5f7a0518bdc7c
Reviewed-on: https://github.com/flang-compiler/f18/pull/429