Doug Gregor b88a875608 [Type checker] Allow extensions of typealiases naming generic specializations. ...

When a (non-generic) typealias refers to a specialization of a generic
type, e.g.

```swift
  typealias simd_float3 = SIMD3<Float>
```

treat an extension of the typealias as an extension of the underlying
type with same-type constraints between the generic parameters and the
specific arguments, e.g.,

```swift
  extension simd_float3 { }
```

is treated as

```swift
  extension SIMD where Scalar == Float { }
```

This addresses a source-compatibility problem with SE-0229, where
existing types such as simd3_float (which were separate structs)
became specializations of a generic SIMD type.

Fixes rdar://problem/46604664 and rdar://problem/46604370.

2018-12-14 13:31:15 -08:00

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