Detect and diagnose a problem when explicitly specified closure
result type doesn't match what is expected by the context:
Example:
```swift
func foo(_: () -> Int) {}
foo { () -> String in "" } // `Int` vs. `String`
```
Let's keep track of type mismatch between type deduced
for the body of the closure vs. what is requested
contextually, it makes it much easier to diagnose
problems like:
```swift
func foo(_: () -> Int) {}
foo { "hello" }
```
Because we can pin-point problematic area of the source
when the rest of the system is consistent.
Resolves: rdar://problem/40537960
When we have a Substring-to-String conversion as a result of
subscripting in a context expecting String, we emit the rather generic
and unactionable availability diagnostic.
Instead, to aid in migration, emit a highly specialized diagnostic and
fixit when we're compiling for -swift-version 4.
