[SILGen] Switch @_extern(c) over to a foreign entrypoint

Rather than treating `@_extern(c)` functions like a Swift function with
a C calling convention, treat them as foreign entrypoints, which
better matches how we handle imported C functions.

Author: DougGregor

lib/SIL/IR/SILDeclRef.cpp

@@ -111,6 +111,9 @@ bool swift::requiresForeignEntryPoint(ValueDecl *vd) {
   if (vd->hasClangNode())
     return true;
 
+  if (ExternAttr::find(vd->getAttrs(), ExternKind::C))
+    return true;
+
   if (auto *accessor = dyn_cast<AccessorDecl>(vd)) {
     // Property accessors should be generated alongside the property.
     if (accessor->isGetterOrSetter()) {

lib/SIL/IR/SILFunctionType.cpp

@@ -4474,8 +4474,6 @@ TypeConverter::getDeclRefRepresentation(SILDeclRef c) {
     case SILDeclRef::Kind::Func:
       if (c.getDecl()->getDeclContext()->isTypeContext())
         return SILFunctionTypeRepresentation::Method;
-      if (ExternAttr::find(c.getDecl()->getAttrs(), ExternKind::C))
-        return SILFunctionTypeRepresentation::CFunctionPointer;
       return SILFunctionTypeRepresentation::Thin;
 
     case SILDeclRef::Kind::Destroyer:

test/SILGen/extern_c.swift

@@ -6,44 +6,44 @@
 @_extern(c, "my_c_name")
 func withCName(_ x: Int) -> Int
 
-// CHECK-DAG: sil hidden_external [asmname "take_c_func_ptr"] @$s8extern_c12takeCFuncPtryyS2iXCF : $@convention(c) (@convention(c) (Int) -> Int) -> ()
+// CHECK-DAG: sil hidden_external [asmname "take_c_func_ptr"] @$s8extern_c12takeCFuncPtryyS2iXCFTo : $@convention(c) (@convention(c) (Int) -> Int) -> ()
 @_extern(c, "take_c_func_ptr")
 func takeCFuncPtr(_ f: @convention(c) (Int) -> Int)
 
-// CHECK-DAG: sil [asmname "public_visible"] @$s8extern_c16publicVisibilityyS2iF : $@convention(c) (Int) -> Int
+// CHECK-DAG: sil [serialized] [asmname "public_visible"] @$s8extern_c16publicVisibilityyS2iFTo : $@convention(c) (Int) -> Int
 @_extern(c, "public_visible")
 public func publicVisibility(_ x: Int) -> Int
 
 // CHECK-DAG: sil [asmname "private_visible"] @$s8extern_c17privateVisibility{{.*}} : $@convention(c) (Int) -> Int
 @_extern(c, "private_visible")
 private func privateVisibility(_ x: Int) -> Int
 
-// CHECK-DAG: sil hidden_external [asmname "withoutCName"] @$s8extern_c12withoutCNameSiyF : $@convention(c) () -> Int
+// CHECK-DAG: sil hidden_external [asmname "withoutCName"] @$s8extern_c12withoutCNameSiyFTo : $@convention(c) () -> Int
 @_extern(c)
 func withoutCName() -> Int
 
 // CHECK-DAG: sil hidden [ossa] @$s8extern_c10defaultArgyySiFfA_ : $@convention(thin) () -> Int {
-// CHECK-DAG: sil hidden_external [asmname "default_arg"] @$s8extern_c10defaultArgyySiF : $@convention(c) (Int) -> ()
+// CHECK-DAG: sil hidden_external [asmname "default_arg"] @$s8extern_c10defaultArgyySiFTo : $@convention(c) (Int) -> ()
 @_extern(c, "default_arg")
 func defaultArg(_ x: Int = 42)
 
 func main() {
   // CHECK-DAG: [[F1:%.+]] = function_ref @$s8extern_c9withCNameyS2iFTo : $@convention(c) (Int) -> Int
-  // CHECK-DAG: [[F2:%.+]] = function_ref @$s8extern_c12takeCFuncPtryyS2iXCF : $@convention(c) (@convention(c) (Int) -> Int) -> ()
+  // CHECK-DAG: [[F2:%.+]] = function_ref @$s8extern_c12takeCFuncPtryyS2iXCFTo : $@convention(c) (@convention(c) (Int) -> Int) -> ()
   // CHECK-DAG: apply [[F2]]([[F1]]) : $@convention(c) (@convention(c) (Int) -> Int) -> ()
   takeCFuncPtr(withCName)
-  // CHECK-DAG: [[F3:%.+]] = function_ref @$s8extern_c16publicVisibilityyS2iF : $@convention(c) (Int) -> Int
+  // CHECK-DAG: [[F3:%.+]] = function_ref @$s8extern_c16publicVisibilityyS2iFTo : $@convention(c) (Int) -> Int
   // CHECK-DAG: apply [[F3]]({{.*}}) : $@convention(c) (Int) -> Int
   _ = publicVisibility(42)
   // CHECK-DAG: [[F4:%.+]] = function_ref @$s8extern_c17privateVisibility{{.*}} : $@convention(c) (Int) -> Int
   // CHECK-DAG: apply [[F4]]({{.*}}) : $@convention(c) (Int) -> Int
   _ = privateVisibility(24)
-  // CHECK-DAG: [[F5:%.+]] = function_ref @$s8extern_c12withoutCNameSiyF : $@convention(c) () -> Int
+  // CHECK-DAG: [[F5:%.+]] = function_ref @$s8extern_c12withoutCNameSiyFTo : $@convention(c) () -> Int
   // CHECK-DAG: apply [[F5]]() : $@convention(c) () -> Int
   _ = withoutCName()
   // CHECK-DAG: [[F6:%.+]] = function_ref @$s8extern_c10defaultArgyySiFfA_ : $@convention(thin) () -> Int
   // CHECK-DAG: [[DEFAULT_V:%.+]] = apply [[F6]]() : $@convention(thin) () -> Int
-  // CHECK-DAG: [[F7:%.+]] = function_ref @$s8extern_c10defaultArgyySiF : $@convention(c) (Int) -> ()
+  // CHECK-DAG: [[F7:%.+]] = function_ref @$s8extern_c10defaultArgyySiFTo : $@convention(c) (Int) -> ()
   // CHECK-DAG: apply [[F7]]([[DEFAULT_V]]) : $@convention(c) (Int) -> ()
   defaultArg()
 }