IRGen: Don't drop method descriptors for vtable-elided internal methods.

We still use the method descriptor for key path identity and other uses. Fixes rdar://problem/66280170.

Author: jckarter

lib/IRGen/GenMeta.cpp

@@ -1492,7 +1492,7 @@ namespace {
     void addMethod(SILDeclRef fn) {
       if (!VTable || methodRequiresReifiedVTableEntry(IGM, VTable, fn)) {
         VTableEntries.push_back(fn);
-      } else if (hasPublicVisibility(fn.getLinkage(NotForDefinition))) {
+      } else {
         // Emit a stub method descriptor and lookup function for nonoverridden
         // methods so that resilient code sequences can still use them.
         emitNonoverriddenMethod(fn);
@@ -1634,7 +1634,10 @@ namespace {
       // method for external clients.
 
       // Emit method dispatch thunk.
-      IGM.emitDispatchThunk(fn);
+      if (hasPublicVisibility(fn.getLinkage(NotForDefinition))) {
+        IGM.emitDispatchThunk(fn);
+      }
+      
       // Emit a freestanding method descriptor structure. This doesn't have to
       // exist in the table in the class's context descriptor since it isn't
       // in the vtable, but external clients need to be able to link against the

test/IRGen/vtable_non_overridden.sil

@@ -24,6 +24,10 @@ sil_vtable InternalA {
   #InternalA.bas : @InternalA_bas [nonoverridden]
 }
 
+// -- we should still generate method descriptors for the elided methods
+// CHECK-LABEL: @"$s21vtable_non_overridden9InternalAC3fooyyFTq" =
+// CHECK-LABEL: @"$s21vtable_non_overridden9InternalAC3basyyFTq" =
+
 // -- only overridden entries in internal method descriptor table
 // CHECK-LABEL: @"$s21vtable_non_overridden9InternalACMn" =
 // CHECK-SAME:     i32 2, %swift.method_descriptor