[IRGen] Added LinkEntity for partial apply forwarder.

Up to now, there had been no need to define a LinkEntity for a partial
apply forwarder.  Now that async partial apply forwarders will each have
their own async function pointer, an entity is needed to pass to the
code that generates the async function pointers.

No demangling or remangling changes are required because that code has
existed for as long as partial apply forwarders to support demangling
their symbols.

Author: nate-chandler

include/swift/IRGen/Linking.h

@@ -447,6 +447,11 @@ class LinkEntity {
     /// context.
     /// The pointer is a SILFunction*.
     AsyncFunctionPointer,
+
+    /// The thunk provided for partially applying a function at some values
+    /// which are captured.
+    /// The pointer is an llvm::Function*.
+    PartialApplyForwarder,
   };
   friend struct llvm::DenseMapInfo<LinkEntity>;
 
@@ -1217,6 +1222,15 @@ class LinkEntity {
     return entity;
   }
 
+  static LinkEntity forPartialApplyForwarder(llvm::Function *function) {
+    LinkEntity entity;
+    entity.Pointer = function;
+    entity.SecondaryPointer = nullptr;
+    entity.Data =
+        LINKENTITY_SET_FIELD(Kind, unsigned(Kind::PartialApplyForwarder));
+    return entity;
+  }
+
   void mangle(llvm::raw_ostream &out) const;
   void mangle(SmallVectorImpl<char> &buffer) const;
   std::string mangleAsString() const;

lib/IRGen/Linking.cpp

@@ -460,6 +460,10 @@ std::string LinkEntity::mangleAsString() const {
     Result.append("Tu");
     return Result;
   }
+  case Kind::PartialApplyForwarder:
+    std::string Result;
+    Result = std::string(static_cast<llvm::Function *>(Pointer)->getName());
+    return Result;
   }
   llvm_unreachable("bad entity kind!");
 }
@@ -740,6 +744,8 @@ SILLinkage LinkEntity::getLinkage(ForDefinition_t forDefinition) const {
   case Kind::DispatchThunkAllocatorAsyncFunctionPointer:
     return getUnderlyingEntityForAsyncFunctionPointer()
         .getLinkage(ForDefinition);
+  case Kind::PartialApplyForwarder:
+    return SILLinkage::Private;
   }
   llvm_unreachable("bad link entity kind");
 }
@@ -821,6 +827,7 @@ bool LinkEntity::isContextDescriptor() const {
   case Kind::NoncanonicalSpecializedGenericTypeMetadata:
   case Kind::NoncanonicalSpecializedGenericTypeMetadataCacheVariable:
   case Kind::CanonicalPrespecializedGenericTypeCachingOnceToken:
+  case Kind::PartialApplyForwarder:
     return false;
   }
   llvm_unreachable("invalid descriptor");
@@ -937,6 +944,8 @@ llvm::Type *LinkEntity::getDefaultDeclarationType(IRGenModule &IGM) const {
   case Kind::DispatchThunkAllocatorAsyncFunctionPointer:
   case Kind::AsyncFunctionPointerAST:
     return IGM.AsyncFunctionPointerTy;
+  case Kind::PartialApplyForwarder:
+    return IGM.FunctionPtrTy;
   default:
     llvm_unreachable("declaration LLVM type not specified");
   }
@@ -992,6 +1001,7 @@ Alignment LinkEntity::getAlignment(IRGenModule &IGM) const {
   case Kind::DifferentiabilityWitness:
   case Kind::NoncanonicalSpecializedGenericTypeMetadata:
   case Kind::NoncanonicalSpecializedGenericTypeMetadataCacheVariable:
+  case Kind::PartialApplyForwarder:
     return IGM.getPointerAlignment();
   case Kind::CanonicalPrespecializedGenericTypeCachingOnceToken:
   case Kind::TypeMetadataDemanglingCacheVariable:
@@ -1086,6 +1096,7 @@ bool LinkEntity::isWeakImported(ModuleDecl *module) const {
   // TODO: Revisit some of the below, for weak conformances.
   case Kind::ObjCMetadataUpdateFunction:
   case Kind::ObjCResilientClassStub:
+  case Kind::PartialApplyForwarder:
   case Kind::TypeMetadataPattern:
   case Kind::TypeMetadataInstantiationCache:
   case Kind::TypeMetadataInstantiationFunction:
@@ -1221,6 +1232,7 @@ DeclContext *LinkEntity::getDeclContextForEmission() const {
   case Kind::ValueWitness:
   case Kind::ValueWitnessTable:
   case Kind::DifferentiabilityWitness:
+  case Kind::PartialApplyForwarder:
     return nullptr;
 
   case Kind::AsyncFunctionPointer: