My recent refactoring of witnesses in Sema and the AST disabled a
test for property behaviors, because they were no longer handling
substitutions correctly. Introduce a type checker entrypoint to record
information about a known witness, using the normal witness-matching
logic rather than trying to synthesize the correct answer (and getting
it wrong).
Note that I had to manually introduce some type witnesses to get the
property behavior tests passing, because the property-behavior code is
(intentionally) not introducing implicit typealiases for the type
witnesses it synthesized. The old witness-synthesizing code worked
around this issue, but the new code does not. A different fix is in
the works (i.e., better handling of type witnesses in the constraint
solver), so we'll take this temporary regression in an experimental
feature.
If behaviors are specified after the declaration, something like this:
```swift
var x: Int __behavior foo // __behavior is a stand-in keyword
```
we're thinking this encourages a simpler design for smaller, more composable behaviors. If we think of behavior application as function-like, then parameters to the behavior could be passed with function-like syntax:
```swift
__behavior lazy(@autoclosure initialValue: () -> Value) { ... }
var x: Int __behavior lazy(1738)
__behavior didSet(body: (oldValue: Value) -> Void) { ... }
var x: Int __behavior didSet {
trailingClosure()
}
```
Since behaviors are implementation details, they arguably belong to the right of the declaration as well.
If a behavior protocol requires an `initialValue` static property, satisfy the requirement using the initial value expression from the property declaration. This lets us implement `lazy` as a property behavior.
Since the feature is incomplete and yet to be accepted or implemented as proposed, hide it behind an -enable-experimental-property-behaviors frontend flag.
Fix some interface type/context type confusion in the AST synthesis from the previous patch, add a unique private mangling for behavior protocol conformances, and set up SILGen to emit the conformances when property declarations with behaviors are visited. Disable synthesis of the struct memberwise initializer if any instance properties use behaviors; codegen will need to be redesigned here.
