[PropertyWrapper] Fix a bug with static property wrapper being rejected in a class

[theblixguy]

We were rejecting the following valid code:

@propertyWrapper
struct Foo<Value> {
  var wrappedValue: Value
}

class Bar {
  // error: class stored properties not supported in classes; did you mean 'static'?
  @Foo static var bool: Bool = true
}

The problem here was that we were using the backing variable's static spelling, which is not present (because we haven't created its pattern binding at the time we access the spelling), so the check was falling down to calling getCorrectStaticSpellingForDecl() which returned class as the spelling since the context is a class.

We should be using the original variable's static spelling instead (we already use its static-ness to decide whether the backing variable should be static or not, might as well use its static spelling too).

Resolves SR-11478.

[jrose-apple]

Nice that this is so small!

[theblixguy]

@swift-ci please smoke test

[DougGregor]

This is great! Can you create a pull request against swift-5.1-branch?

[theblixguy]

@DougGregor Sure, here you go: #27212