Add minimal SILGen and Serialization tests for variadic generics

Author: slavapestov

test/SILGen/pack_expansion_type.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-emit-silgen %s -enable-experimental-feature VariadicGenerics | %FileCheck %s
+
+// CHECK-LABEL: sil [ossa] @$s19pack_expansion_type16variadicFunction1t1ux_q_txQp_txxQp_q_q_Qptq_Rhzr0_lF : $@convention(thin) <T..., U... where ((T, U)...) : Any> (@in_guaranteed T..., @in_guaranteed U...) -> @out (T, U)... {
+// CHECK: bb0(%0 : $*(T, U)..., %1 : $*T..., %2 : $*U...):
+public func variadicFunction<T..., U...>(t: T..., u: U...) -> ((T, U)...) {}
+
+public struct VariadicType<T...> {
+  // CHECK-LABEL: sil [ossa] @$s19pack_expansion_type12VariadicTypeV14variadicMethod1t1ux_qd__txQp_txxQp_qd__qd__Qptqd__RhzlF : $@convention(method) <T...><U... where ((T, U)...) : Any> (@in_guaranteed T..., @in_guaranteed U..., VariadicType<T>) -> @out (T, U)... {
+  // CHECK: bb0(%0 : $*(T, U)..., %1 : $*T..., %2 : $*U..., %3 : $VariadicType<T>):
+  public func variadicMethod<U...>(t: T..., u: U...) -> ((T, U)...) {}
+}

test/Serialization/Inputs/pack_expansion_type_other.swift

@@ -0,0 +1,11 @@
+public func variadicFunction<T..., U...>(t: T..., u: U...) -> ((T, U)...) {
+  // FIXME: return ((t, u)...)
+  fatalError()
+}
+
+public struct VariadicType<T...> {
+  public func variadicMethod<U...>(t: T..., u: U...) -> ((T, U)...) {
+    // FIXME: return ((t, u)...)
+    fatalError()
+  }
+}

test/Serialization/pack_expansion_type.swift

@@ -0,0 +1,8 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %S/Inputs/pack_expansion_type_other.swift -emit-module -emit-module-path %t/pack_expansion_type_other.swiftmodule -enable-experimental-feature VariadicGenerics
+
+import pack_expansion_type_other
+
+variadicFunction(t: 1, 2, u: "hi", "bye")
+
+VariadicType<Int, String>.variadicMethod(t: 1, 2, u: "hi", "bye")