[5.1] [NFC] Correct assumptions about static AbstractStorageDecls

[beccadax]

Cherry-pick of #23724 to swift-5.1-branch. The master PR was reviewed by @slavapestov and @xedin.

This PR contains some of the refactoring needed to support static subscripts:

• Modifies AbstractStorageDecl and its subclasses to allow the AST to express the idea of a static subscript.
• Modifies various parts of the compiler to avoid casting from AbstractStorageDecl to VarDecl before checking staticness. These changes are mostly mechanical, but I've nontrivially refactored one place in override checking.
• Corrects a place in the constraint solver which assumes the self parameter of a subscript is never a metatype.

It does not modify the parser to accept static modifiers on subscripts, nor does it modify the type checker to allow subscripts to be applied to metatypes, so there should not be a way to create or use a static subscript. It also tries not to add much new code which will only be exercised when handling a static subscript.

I'd like to land these changes early to give them a little more time to bake and to avoid repeated merge conflicts, particularly on the serialization version number. This PR by itself should not cause any behavior changes; #23358, which implements static subscripts, includes tests verifying that the relevant features behave correctly with static subscripts.

[beccadax]

@swift-ci please test

[beccadax]

@swift-ci please test