[NFC] Light refactoring of ParamDecl

[CodaFi]

The calm before the storm.

This patch involves 3 simple transformations:

• Now that we have a richer representation of parameter specifiers, code synthesis can run through the same argument creation path without fear of boolean blindness.

• Specifier::None is renamed to Specifier::Owned in anticipation of staging

• setLet is replaced by the more general setSpecifier

[CodaFi]

@swift-ci please smoke test and merge

[CodaFi]

Whoops, forgot some missing InOutType::gets after the cherry pick.

@swift-ci please smoke test and merge

[jrose-apple]

Specifier::None is renamed to Specifier::Owned in anticipation of staging

Are specifiers syntactic or semantic? If they're semantic, differentiating between Let and Owned doesn't seem useful.

[CodaFi]

In this specific case it's syntactic. When owned becomes a specifier for var decls then we can collapse the two.

[jrose-apple]

Are we expecting owned to become a specifier for var decls?

[jrose-apple]

We also often do find it useful to track how the user wrote something. I'm not convinced that it's a good idea to make this change.

[CodaFi]

The ownership manifesto wants it, but I'm treating it as out of scope as it relates to the work I'm doing.