This will unblock parsing and type-checking availability queries that specify
custom availability domains, e.g.:
```
if #available(CustomDomain) {
// Use declarations protected by @available(CustomDomain)
}
```
It wraps an type-checked `AvailabilitySpec`, which guarantees that the spec has
a valid `AvailabilityDomain` associated with it. This will unblock moving
AvailabilitySpec domain resolution from parsing to sema.
Instead of canonicalizing platform versions during parsing and storing two
versions, just canonicalize the parsed version on-demand when its requested.
* Move `AvailabilitySpec` handling logic to AST, so they can be shared
between libParse and ASTGen
* Requestify '-define-availability' arguments parsing and parse them
with 'SwiftParser' according to the 'ParserASTGen' feature flag
* Implement 'AvailableAttr' generation in ASTGen
