Simplify and test ASTGen support

Rather than adding custom parsing to SwiftSyntax, we can parse this as a custom attribute and convert it to a built-in one in ASTGen. Test that this works correctly (and fix a bug where it wasn’t).

Author: beccadax

lib/AST/ASTDumper.cpp

@@ -1084,6 +1084,16 @@ namespace {
 
       printFlag(D->isImplicit(), "implicit", DeclModifierColor);
       printFlag(D->isHoisted(), "hoisted", DeclModifierColor);
+
+      if (auto implAttr = D->getAttrs().getAttribute<ObjCImplementationAttr>()) {
+        StringRef label =
+            implAttr->isEarlyAdopter() ? "objc_impl" : "clang_impl";
+        if (implAttr->CategoryName.empty())
+          printFlag(label);
+        else
+          printFieldQuoted(implAttr->CategoryName.str(), label);
+      }
+
       printSourceRange(D->getSourceRange(), &D->getASTContext());
       printFlag(D->TrailingSemiLoc.isValid(), "trailing_semi",
                 DeclModifierColor);

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -1066,6 +1066,11 @@ extension ASTGenVisitor {
       return nil
     }
 
+    if case .token(let tok) = arguments {
+      // Special case: was parsed as a token, not an an argument list
+      return valueGeneratorFunction(tok)
+    }
+
     guard var arguments = arguments.as(LabeledExprListSyntax.self)?[...] else {
       // TODO: Diagnose.
       return nil

test/ASTGen/attrs.swift

@@ -57,3 +57,8 @@ func testMutating(value: S3) {
 class C1 {}
 @_alignment(7) // expected-error {{alignment value must be a power of two}}
 struct S4 {}
+
+@implementation extension ObjCClass1 {} // expected-error {{cannot find type 'ObjCClass1' in scope}}
+@implementation(Category) extension ObjCClass1 {} // expected-error {{cannot find type 'ObjCClass1' in scope}}
+@_objcImplementation extension ObjCClass2 {} // expected-error {{cannot find type 'ObjCClass2' in scope}}
+@_objcImplementation(Category) extension ObjCClass2 {} // expected-error {{cannot find type 'ObjCClass2' in scope}}