[IRGen] Give decl ref to LinkEntity::forAFP.

Previously, a LinkEntity for an AST async function pointer was built by
passing an AbstractFunctionDecl.  Later, decl was used to construct a
SILDeclRef.

That arrangement meant that clients could not construct such a
LinkEntity whose SILDeclRef::Kind could not be inferred from the dynamic
type of the decl from which the SILDeclRef was constructed.  In
particular, clients could not construct a LinkEntity for the initializer
corresponding to a ConstructorDecl.

Here, the arrangment is changed so that the LinkEntity for an AST async
function pointer is built by passing a SILDeclRef.

Author: nate-chandler

include/swift/IRGen/Linking.h

@@ -1196,9 +1196,12 @@ class LinkEntity {
     return entity;
   }
 
-  static LinkEntity forAsyncFunctionPointer(AbstractFunctionDecl *decl) {
+  static LinkEntity forAsyncFunctionPointer(SILDeclRef declRef) {
     LinkEntity entity;
-    entity.setForDecl(Kind::AsyncFunctionPointerAST, decl);
+    entity.setForDecl(Kind::AsyncFunctionPointerAST,
+                      declRef.getAbstractFunctionDecl());
+    entity.SecondaryPointer =
+        reinterpret_cast<void *>(static_cast<uintptr_t>(declRef.kind));
     return entity;
   }
 

lib/IRGen/Linking.cpp

@@ -462,7 +462,10 @@ std::string LinkEntity::mangleAsString() const {
 
   case Kind::AsyncFunctionPointerAST: {
     std::string Result;
-    Result = SILDeclRef(const_cast<ValueDecl *>(getDecl())).mangle();
+    Result = SILDeclRef(const_cast<ValueDecl *>(getDecl()),
+                        static_cast<SILDeclRef::Kind>(
+                            reinterpret_cast<uintptr_t>(SecondaryPointer)))
+                 .mangle();
     Result.append("Tu");
     return Result;
   }