Add @below and @above directives to verify-diagnostics.

This simplifies defining expected-* directives when there are multiple that apply to the next or previous line. @below applies the directive to the next non-designator line, i.e. the next line that does not contain an expected-* designator. @above applies to the previous non designator line.

Examples:

// Expect an error on the next line that does not contain a designator.
// expected-remark@below {{remark on function below}}
// expected-remark@below {{another remark on function below}}
func @bar(%a : f32)

// Expect an error on the previous line that does not contain a designator.
func @baz(%a : f32)
// expected-remark@above {{remark on function above}}
// expected-remark@above {{another remark on function above}}

PiperOrigin-RevId: 276369085