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

[theblixguy]

Cherry-picking #27198. The first commit is new because we don't have TypeCheckStorage.cpp in the 5.1 branch. The second commit is from the original PR.

Explanation: 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).

Scope: Property wrapper usage inside a class

Issue: SR-11478

Testing: Swift CI

Risk: Low. This allows code that was previously rejected (incorrectly)

Reviewed by: @jrose-apple

[theblixguy]

@swift-ci please test

[DougGregor]

@swift-ci please nominate

[theblixguy]

Uh oh, a merge conflict sneaked in! Rerunning CI...

@swift-ci please test

[DougGregor]

Drat, the conflict is probably from the merge of your other 5.1 PR. Thanks for fixing the conflict!