This includes changing the feature name so that compilers with the experimental feature don’t accidentally pick up content that only works in the final version.
Resolves rdar://150065196.
The new `DECL_ATTR_FEATURE_REQUIREMENT` macro in DeclAttr.def can be used to declare that an attribute should only be available when a related language feature is enabled.
Effects:
• `#if hasAttribute(someAttr)` will return `false` unless the required feature is enabled.
• Code completion will not include the attribute unless the required feature is enabled.
• `TypeChecker::checkDeclAttributes()` diagnoses non-implicit uses of the attribute.
Add this mechanism and use it to tie @abi to the ABIAttribute feature. Also design tests for it.
