[InterfacePrinting] Add a print option to specify whether members of a decl should be separated by empty new lines.

By doing so, SourceKit can print Swift intefaces in the same fashion of what Xcode conventionally does.
rdar://21710381

Swift SVN r30430

include/swift/AST/PrintOptions.h

@@ -144,6 +144,9 @@ struct PrintOptions {
   /// Whether we are printing part of SIL body.
   bool PrintInSILBody = false;
 
+  /// Whether to use an empty line to separate two members in a single decl.
+  bool EmptyLineBetweenMembers = false;
+
   enum class ArgAndParamPrintingMode {
     ArgumentOnly,
     MatchSource,

lib/AST/ASTPrinter.cpp

@@ -222,7 +222,6 @@ class PrintAST : public ASTVisitor<PrintAST> {
     if (RC.isEmpty())
       return;
 
-    Printer.printNewline();
     indent();
 
     SmallVector<StringRef, 8> Lines;
@@ -885,6 +884,8 @@ void PrintAST::printMembers(DeclRange members, bool needComma) {
       if (!member->shouldPrintInContext(Options))
         continue;
 
+      if (Options.EmptyLineBetweenMembers)
+        Printer.printNewline();
       indent();
       visit(member);
       if (needComma && std::next(i) != iEnd)
@@ -1691,7 +1692,6 @@ void PrintAST::visitEnumCaseDecl(EnumCaseDecl *decl) {
 void PrintAST::visitEnumElementDecl(EnumElementDecl *decl) {
   if (!decl->shouldPrintInContext(Options))
     return;
-
   printDocumentationComment(decl);
   // In cases where there is no parent EnumCaseDecl (such as imported or
   // deserialized elements), print the element independently.

lib/AST/Module.cpp

@@ -1390,16 +1390,12 @@ void SourceFile::print(ASTPrinter &Printer, const PrintOptions &PO) {
   for (auto decl : Decls) {
     if (!decl->shouldPrintInContext(PO))
       continue;
-
-    if(decl->print(Printer, PO)) {
+    // For a major decl, we print an empty line before it.
+    if (MajorDeclKinds.find(decl->getKind()) != MajorDeclKinds.end())
       Printer << "\n";
-
-      // For a major decl, we print an empty line after it.
-      if (MajorDeclKinds.find(decl->getKind()) != MajorDeclKinds.end()) {
+    if(decl->print(Printer, PO))
       Printer << "\n";
   }
-    }
-  }
 }
 
 void SourceFile::addImports(

test/IDE/Inputs/foo_swift_module.printed.comments.txt

@@ -20,14 +20,12 @@ protocol BarProtocol {
   func instanceFunc()
 }
 
-
 /// FooSwiftStruct Aaa.
 /**
  * Bbb.
  * Ccc.
  */
 struct FooSwiftStruct {
-
   /// fooInstanceFunc Aaa.
   /**
    * Bbb
@@ -39,10 +37,8 @@ struct FooSwiftStruct {
   init()
 }
 
-
 /// rdar://18457785
 enum MyQuickLookObject {
-
   /// A rectangle.
   ///
   /// Uses explicit coordinates to avoid coupling a particular Cocoa type.