[PropertyWrappers] Mark the property as invalid when there is a mismatch with wrappedValue type

[theblixguy]

When the property's type does not match the wrappedValue property's type, mark the property as invalid. Otherwise, we will end up producing a bunch of incorrect diagnostics later on as well as crashing in certain cases.

For example, this little snippet crashes the compiler:

@propertyWrapper 
struct Wrapper {
  var wrappedValue: Int
}

class Test {
  @Wrapper var user: Int?
}

because we're trying to synthesise a setter that assigns a value of type Int? into a storage of type Int. We also end up emitting a bunch of incorrect diagnostics, such as suggesting the user to force unwrap the property declaration or provide a default value (which isn't possible/make sense).

In addition, we emit another diagnostic about failing to convert the argument type, for the synthesised wrappedValue initializer. This diagnostic isn't valuable because (1) we already inform the user about the type mismatch between the property's type and the wrappedValue type and (2) the diagnostic is attached to the synthesised init, but appears on the property and can be confusing to the user. This diagnostic is now disabled for a synthesised wrappedValue init.

Resolves SR-11684
Resolves SR-11600
Resolves rdar://problem/56785162
Resolves rdar://problem/56345248
Resolves rdar://problem/56190606

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@DougGregor can you take a quick look at this? I'd like to merge this before tomorrow's cutoff. Thank you!

[theblixguy]

I just noticed that this fixes compiler crasher SR-11600 as well. Great! The diagnostics can definitely be improved for wrapper composition though (mostly its the diagnostics around the synthesised wrappedValue init that needs a bit of special-casing/tweaking).

[theblixguy]

@swift-ci please smoke test

[DougGregor]

Thank you!