[flang] Add SayWithDecl function for error reporting

Add `SayWithDecl` to handle the common case of reporting an error about
a symbol and referencing its original declaration.

Original-commit: flang-compiler/f18@cf4d6fbe7f6b977f8b683a37a47109f642e2a44c
Reviewed-on: https://github.com/flang-compiler/f18/pull/277
Tree-same-pre-rewrite: false