swift

b88a8756 - [Type checker] Allow extensions of typealiases naming generic specializations.

[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.