Fixe a couple of bugs in libSyntax parsing found by enabling `-verify-syntax-tree` for `%target-build-swift`:
- Fix parsing of the `actor` contextual keyword in actor decls
- Don't build a libSyntax tree when parsing the availability macro
- The availability macro is not part of the source code and doesn't form a valid Swift file, thus creation of a libSyntax tree is completely pointless and will fail
- Add support for parsing `@_originallyDefinedIn` attributes.
- Add support for parsing `#sourceLocation` in member decl lists
- Add support for effectful properties (throwing/async getters/setters)
- Add support for optional types as the base of a key path (e.g. `\TestOptional2?.something`)
- Allow platform restrictions without a version (e.g. `_iOS13Aligned`)
