Correct newline emission in generated headers

Eliminates extraneous newlines between top-level Objective-C declarations in `-emit-objc-header` headers. Specifically, there should now always be exactly one—no more, no less—empty line between `@end` and whatever follows it.

Besides being more aesthetically pleasing, this eliminates ordering-dependent behavior where PrintAsClang would print an extra newline when visiting an empty extension, which meant that the order in which empty and non-empty extensions were visited during printing could result in whitespace differences in the compiler output. Printing the blank line is now conditional on whether `tell()` indicates that characters were actually written to the output.

Fixes rdar://143533893.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -488,7 +488,6 @@ public:
         (inserted || !it->second.second))
       ClangValueTypePrinter::forwardDeclType(os, CD, printer);
     it->second = {EmissionState::Defined, true};
-    os << '\n';
     printer.print(CD);
     return true;
   }
@@ -507,7 +506,6 @@ public:
           forwardDeclareType(TD);
         });
 
-    os << '\n';
     printer.print(FD);
     return true;
   }
@@ -550,7 +548,6 @@ public:
       return false;
 
     seenTypes[PD] = { EmissionState::Defined, true };
-    os << '\n';
     printer.print(PD);
     return true;
   }
@@ -576,7 +573,6 @@ public:
     if (!forwardDeclareMemberTypes(ED->getAllMembers(), ED))
       return false;
 
-    os << '\n';
     printer.print(ED);
     return true;
   }
@@ -840,6 +836,7 @@ public:
     while (!declsToWrite.empty()) {
       const Decl *D = declsToWrite.back();
       bool success = true;
+      auto posBefore = os.tell();
 
       if (auto ED = dyn_cast<EnumDecl>(D)) {
         success = writeEnum(ED);
@@ -870,7 +867,10 @@ public:
 
       if (success) {
         assert(declsToWrite.back() == D);
-        os << "\n";
+        // If we actually wrote something to the file, add a newline after it.
+        // (As opposed to, for instance, an extension we decided to skip.)
+        if (posBefore != os.tell())
+          os << "\n";
         declsToWrite.pop_back();
       }
     }