[ModuleInterface] Allow conformances to be missing value witnesses (#18932)

It's not clear whether we'll actually need this feature in the long
run, but we certainly need it now because non-@usableFromInline
members can (currently) satisfy public requirements when a
@usableFromInline internal type conforms to a public protocol. In
these cases, we'll treat the witnesses as present but opaque, and
clients will perform dynamic dispatch when using them even when
a generic function gets specialized.

With this, we're able to generate a textual interface for the standard
library, compile it back to a swiftmodule, and use it to build a Hello
World program!

test/ModuleInterface/Conformances.swiftinterface

@@ -0,0 +1,34 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -enable-resilience -o %t/Conformances.swiftmodule %s
+// RUN: %target-swift-frontend -emit-sil -I %t %S/Inputs/ConformancesUser.swift -O | %FileCheck %s
+
+public protocol MyProto {
+  init()
+  func method()
+  var prop: Int { get set }
+  subscript(index: Int) -> Int { get set }
+}
+
+@_fixed_layout // allow conformance devirtualization
+public struct FullStructImpl: MyProto {
+  public init()
+  public func method()
+  public var prop: Int { get set }
+  public subscript(index: Int) -> Int { get set }
+}
+// CHECK-LABEL: sil @$S16ConformancesUser8testFullSiyF
+// CHECK: function_ref @$S12Conformances14FullStructImplVACycfC
+// CHECK: function_ref @$S12Conformances14FullStructImplV6methodyyF
+// CHECK: function_ref @$S12Conformances14FullStructImplV4propSivs
+// CHECK: function_ref @$S12Conformances14FullStructImplVyS2icig
+// CHECK: end sil function '$S16ConformancesUser8testFullSiyF'
+
+@_fixed_layout // allow conformance devirtualization
+public struct OpaqueStructImpl: MyProto {}
+
+// CHECK-LABEL: sil @$S16ConformancesUser10testOpaqueSiyF
+// CHECK: function_ref @$S12Conformances7MyProtoPxycfC
+// CHECK: function_ref @$S12Conformances7MyProtoP6methodyyF
+// CHECK: function_ref @$S12Conformances7MyProtoP4propSivs
+// CHECK: function_ref @$S12Conformances7MyProtoPyS2icig
+// CHECK: end sil function '$S16ConformancesUser10testOpaqueSiyF'

test/ModuleInterface/Inputs/ConformancesUser.swift

@@ -0,0 +1,16 @@
+import Conformances
+
+func testGeneric<T: MyProto>(_: T.Type) -> Int {
+  var impl = T.init()
+  impl.method()
+  impl.prop = 0
+  return impl[0]
+}
+
+public func testFull() -> Int {
+  return testGeneric(FullStructImpl.self)
+}
+
+public func testOpaque() -> Int {
+  return testGeneric(OpaqueStructImpl.self)
+}