swift234

[Sema]Allow @availability attributes on extensions

This commit allows @availability attributes on extensions.

Unlike other declarations, extensions can be used without referring to them
by name (they don't have one) in the source. For this reason, when checking
the available version range of a declaration we also need to check to see if it is
immediately contained in an extension and use the extension's availability if
the declaration does not have an explicit @availability attribute itself.

This commit also moves building the primary file type refinement context hierarchy
in performTypeChecking() to before we resolve extensions.  Resolving extensions checks for
availability of the extended declaration, so the TRC for the extension must be constructed
before then.

Swift SVN r25589