The single flag `has_syntax_error` on `LinterResult` is replaced with
two (private) flags: `has_valid_syntax` and
`has_no_unsupported_syntax_errors`, which record whether there are
`ParseError`s or `UnsupportedSyntaxError`s, respectively. Only the
former is used to prevent a `FixAll` action.
An attempt has been made to make consistent the usage of the phrases
"valid syntax" (which seems to be used to refer only to _parser_ errors)
and "syntax error" (which refers to both _parser_ errors and
version-specific syntax errors).
Closes#16841
74f64d3f96