IRGen: Don't emit method descriptors for derivative functions.

The symbol mangling for the descriptors doesn't incorporate the derivative information, leading to symbol collisions.

Author: jckarter

include/swift/IRGen/Linking.h

@@ -575,7 +575,7 @@ class LinkEntity {
   static bool isValidResilientMethodRef(SILDeclRef declRef) {
     if (declRef.isForeign)
       return false;
-
+    
     auto *decl = declRef.getDecl();
     return (isa<ClassDecl>(decl->getDeclContext()) ||
             isa<ProtocolDecl>(decl->getDeclContext()));

lib/IRGen/GenMeta.cpp

@@ -1609,8 +1609,9 @@ namespace {
     }
 
     void emitMethodDescriptor(SILDeclRef fn) {
+
       // Define the method descriptor to point to the current position in the
-      // nominal type descriptor.
+      // nominal type descriptor, if it has a well-defined symbol name.
       IGM.defineMethodDescriptor(fn, Type,
                       B.getAddrOfCurrentPosition(IGM.MethodDescriptorStructTy));
 
@@ -1628,7 +1629,12 @@ namespace {
     }
 
     void emitNonoverriddenMethod(SILDeclRef fn) {
-      HasNonoverriddenMethods = true;
+      // TODO: Derivative functions do not distinguish themselves in the mangled
+      // names of method descriptor symbols yet, causing symbol name collisions.
+      if (fn.derivativeFunctionIdentifier)
+        return;
+
+     HasNonoverriddenMethods = true;
       // Although this method is non-overridden and therefore left out of the
       // vtable, we still need to maintain the ABI of a potentially-overridden
       // method for external clients.