Format_doc: preserve the type of Foo.report_error, add Foo.report_error_doc (#13311)
* Format_doc: preserve the type of Foo.report_error, add Foo.report_error_doc
The introduction of Format_doc changed the type of various
'reprot_error' functions in the compiler codebase. But this breaks
user code. An alternative proposed here is to keep 'report_error'
at the same type as before and introduce 'report_error_doc' in addition.
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