embedded: fix specialization of associated conformance entries in witness tables

When creating a specialized witness table, we need to get the right specialized conformance.
In IRGen don't emit associated conformance witness table entries if the protocol is not a class protocol.
In this case the associated type can never be used to create an existential. Therefore such a witness table entry is never used at runtime in embedded swift.

Fixes a compiler crash

rdar://146448091

test/embedded/existential-class-bound1.swift

@@ -156,6 +156,82 @@ struct S: P2 {
   }
 }
 
+protocol Q3 {
+  func bar()
+}
+
+protocol P3<T>: AnyObject {
+  associatedtype T: Q3
+
+  var t: T { get }
+
+  func foo()
+}
+
+extension P3 {
+  func foo() {
+    t.bar()
+  }
+}
+
+final class C3<T: Q3>: P3 {
+  var t: T
+
+
+  init(t: T) { self.t = t }
+}
+
+struct S3<I: BinaryInteger>: Q3 {
+  var x: I
+
+  func bar() {
+    print(x)
+  }
+}
+
+@inline(never)
+func testP3() -> any P3 {
+  return C3(t: S3(x: 102))
+}
+
+protocol P4<T>: AnyObject {
+  associatedtype T: Q
+
+  var t: T { get }
+
+  func foo()
+}
+
+extension P4 {
+  func foo() {
+    t.bar()
+  }
+}
+
+final class C4<T: Q>: P4 {
+  var t: T
+
+
+  init(t: T) { self.t = t }
+}
+
+class K4<I: BinaryInteger>: Q {
+  var x: I
+
+  init(x: I) { self.x = x }
+
+  func bar() {
+    print(x)
+  }
+}
+
+@inline(never)
+func testP4() -> any P4 {
+  return C4(t: K4(x: 437))
+}
+
+
+
 @main
 struct Main {
     static func main() {
@@ -181,6 +257,10 @@ struct Main {
         // CHECK: Derived2.bar()
         testConditionalConformance(t: S(i: 27))
         // CHECK: 27
+        testP3().foo()
+        // CHECK: 102
+        testP4().foo()
+        // CHECK: 437
     }
 }