gh-107570: Argument Clinic: report errors on the offending line

[serhiy-storchaka]

Errors raised while the docstring is checked were reported on the line which ends the clinic block:

Error in file 'p.c' on line 11:      # the "[clinic start generated code]*/" line
Docstring for 'm.f' does not have a summary line!

and errors raised while the code is generated were reported without a file name and a line number at all:

Error:
You cannot use optional groups ('[' and ']') unless all parameters are positional-only ('/').

Functions and parameters now record the line on which they are declared, and the function docstring records where it starts, so that such errors point at the offending line: the line which should have been left blank, the too long summary line, the first too long line of the docstring body, the second {parameters} marker, or the declaration of the function or the parameter.

• Issue: Argument Clinic reports incorrect line numbers on error #107570