This contains the category name, if there is one.

Several offsetting bugs both broke the caching of `ObjCInterfaceAndImplementationRequest` and caused it to usually miss. Fix this whole painful mess. Also has collateral improvements to simple_display().

This now specifies a category name that’s used in TBDGen, IRGen, and PrintAsClang. There are also now category name conflict diagnostics; these subsume some @implementation diagnostics.

(It turns out there was already a check for @objc(CustomName) to make sure it wasn’t a selector!)

Their syntaxes are about to diverge, so let’s make sure that we maintain source compatibility for @_objcImplementation.

This functionality was previously reserved for ValueDecls. Move it all the way up to Decl; in the process, make it correctly handle EnumElementDecls and EnumCaseDecls.

This change also allows us to generalize `swift::fixDeclarationObjCName()` to work on extensions, though we do not use that capability in this commit.

…for extensions. This change also removes @implementation(CategoryName); you should attach the category name to the @objc attribute instead. And there are small changes to how much checking the compiler will do on an @objc @implementation after the decl checker has discovered a problem with it.

Previous changes have made it so that `private let`s in an objcImpl extension are implicitly `@objc`, which changed downstream behavior in a SILOptimizer test. The new behavior is actually more correct, so codify this behavior change by modifying the test.