[CodeCompletion] Don't attach attr to decl if blank line exists

```
@#^COMPLETE^#

public func something() {}
```
In this case, we can't say the user is adding attribute to the func or
starting a new declaration. So if there're one or more blank lines after the
completion, suggest context free attribute list.

rdar://problem/50441643

Author: rintaro

include/swift/Parse/CodeCompletionCallbacks.h

@@ -189,7 +189,7 @@ class CodeCompletionCallbacks {
   virtual void completeAccessorBeginning(CodeCompletionExpr *E) {};
 
   /// Complete the keyword in attribute, for instance, @available.
-  virtual void completeDeclAttrBeginning(bool Sil) {};
+  virtual void completeDeclAttrBeginning(bool Sil, bool isIndependent) {};
 
   /// Complete the parameters in attribute, for instance, version specifier for
   /// @available.

lib/IDE/CodeCompletion.cpp

@@ -1351,6 +1351,13 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
   void setAttrTargetDeclKind(Optional<DeclKind> DK) override {
     if (DK == DeclKind::PatternBinding)
       DK = DeclKind::Var;
+
+    // If the target is already set to 'Module', that means the completion
+    // should be performed as if it's not tied to any specific decl.
+    // see 'completeDeclAttrBeginning()'
+    if (AttTargetDK == DeclKind::Module)
+      DK = None;
+
     AttTargetDK = DK;
   }
 
@@ -1373,7 +1380,7 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
   void completeCaseStmtKeyword() override;
   void completeCaseStmtBeginning() override;
   void completeCaseStmtDotPrefix() override;
-  void completeDeclAttrBeginning(bool Sil) override;
+  void completeDeclAttrBeginning(bool Sil, bool isIndependent) override;
   void completeDeclAttrParam(DeclAttrKind DK, int Index) override;
   void completeInPrecedenceGroup(SyntaxKind SK) override;
   void completeNominalMemberBeginning(
@@ -4606,10 +4613,15 @@ void CodeCompletionCallbacksImpl::completeDeclAttrParam(DeclAttrKind DK,
   CurDeclContext = P.CurDeclContext;
 }
 
-void CodeCompletionCallbacksImpl::completeDeclAttrBeginning(bool Sil) {
+void CodeCompletionCallbacksImpl::completeDeclAttrBeginning(
+    bool Sil, bool isIndependent) {
   Kind = CompletionKind::AttributeBegin;
   IsInSil = Sil;
   CurDeclContext = P.CurDeclContext;
+
+  // Use 'DeclKind::Module' as the indicator of "This is independent attribute".
+  if (isIndependent)
+    AttTargetDK = DeclKind::Module;
 }
 
 void CodeCompletionCallbacksImpl::completeInPrecedenceGroup(SyntaxKind SK) {
@@ -5373,6 +5385,8 @@ void CodeCompletionCallbacksImpl::doneParsing() {
   }
 
   case CompletionKind::AttributeBegin: {
+    assert(AttTargetDK != DeclKind::Module &&
+           "'setAttrTargetDeclKind()' hasn't been called");
     Lookup.getAttributeDeclCompletions(IsInSil, AttTargetDK);
 
     // TypeName at attribute position after '@'.

lib/Parse/ParseDecl.cpp

@@ -1753,8 +1753,12 @@ ParserStatus Parser::parseDeclAttribute(DeclAttributes &Attributes, SourceLoc At
       Tok.isNot(tok::kw_inout)) {
 
     if (Tok.is(tok::code_complete)) {
-      if (CodeCompletion)
-        CodeCompletion->completeDeclAttrBeginning(isInSILMode());
+      if (CodeCompletion) {
+        auto ccLine = SourceMgr.getLineNumber(Tok.getLoc());
+        auto nextLine = SourceMgr.getLineNumber(peekToken().getLoc());
+        CodeCompletion->completeDeclAttrBeginning(isInSILMode(),
+                                                  (nextLine - ccLine) > 1);
+      }
       consumeToken(tok::code_complete);
       return makeParserCodeCompletionStatus();
     }

test/IDE/complete_decl_attribute.swift

@@ -10,7 +10,9 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ON_PROPERTY | %FileCheck %s -check-prefix=ON_PROPERTY
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ON_METHOD | %FileCheck %s -check-prefix=ON_METHOD
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ON_PARAM | %FileCheck %s -check-prefix=ON_PARAM
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ON_MEMBER_INDEPENDENT | %FileCheck %s -check-prefix=ON_MEMBER_LAST
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ON_MEMBER_LAST | %FileCheck %s -check-prefix=ON_MEMBER_LAST
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD_INDEPENDENT | %FileCheck %s -check-prefix=KEYWORD_LAST
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=KEYWORD_LAST | %FileCheck %s -check-prefix=KEYWORD_LAST
 
 struct MyStruct {}
@@ -160,6 +162,7 @@ struct _S {
 // ON_PROPERTY: End completions
 
   @#^ON_METHOD^#
+  private
   func foo()
 // ON_METHOD: Begin completions
 // ON_METHOD-DAG: Keyword/None:                       available[#Func Attribute#]; name=available
@@ -184,6 +187,10 @@ struct _S {
 // ON_PARAM-NOT: Keyword
 // ON_PARAM: End completions
 
+  @#^ON_MEMBER_INDEPENDENT^#
+
+  func dummy() {}
+
   @#^ON_MEMBER_LAST^#
 // ON_MEMBER_LAST: Begin completions
 // ON_MEMBER_LAST-DAG: Keyword/None:                       available[#Declaration Attribute#]; name=available
@@ -216,6 +223,10 @@ struct _S {
 // ON_MEMBER_LAST: End completions
 }
 
+@#^KEYWORD_INDEPENDENT^#
+
+func dummy() {}
+
 @#^KEYWORD_LAST^#
 
 // KEYWORD_LAST:                  Begin completions