swift234

Sema: Update diagnostic text for when @available cannot be applied to a variable.

This changes the diagnostic text when a variable cannot be marked potentially unavailable
so that it does not explicitly use the long-style form of @available:

It changes:
  "stored properties cannot be marked potentially unavailable with 'introduced='"
to:
  "stored properties cannot be marked potentially unavailable with '@available'"

Swift SVN r30446