Remove an old diagnostic for 'var' in param position

[CodaFi]

Using the attribute in this position is a relic from the Swift 2
days, and fixing it required letting invalid code fall through to
Sema instead of being diagnosed in Parse proper. Treat 'var'
in this position like 'let' by simply offering to remove it
instead of extracting it into a separate variable.

This simplifies our model by making the mapping between parsed parameter specifiers and parameter declaration specifiers one-to-one. In a future commit, these two representations will merge.

This is currently blocking #10695.

[CodaFi]

@swift-ci please smoke test and merge

[CodaFi]

Going to file an SR (starter bug?) about improving this diagnostic a different way. In addition to @jrose-apple's idea to enqueue this diagnostic and waiting until we see the in of a closure to try the extraction, we could also just do what we do for parse and offer to stick an inout on the type in addition to removing the var specifier if we're in function position or we have a type ascription.

[jrose-apple]

I don't think people who write var on a parameter actually mean inout. They just want to do the C thing of using it as a temporary variable as well.