[YAML] fix output incorrect format for block scalar string

Author: HerrCai0907

llvm/lib/Support/YAMLTraits.cpp

@@ -725,18 +725,18 @@ void Output::blockScalarString(StringRef &S) {
   if (!StateStack.empty())
     newLineCheck();
   output(" |");
-  outputNewLine();
 
   unsigned Indent = StateStack.empty() ? 1 : StateStack.size();
 
   auto Buffer = MemoryBuffer::getMemBuffer(S, "", false);
   for (line_iterator Lines(*Buffer, false); !Lines.is_at_end(); ++Lines) {
+    outputNewLine();
     for (unsigned I = 0; I < Indent; ++I) {
       output("  ");
     }
     output(*Lines);
-    outputNewLine();
   }
+  outputUpToEndOfLine("");
 }
 
 void Output::scalarTag(std::string &Tag) {

llvm/unittests/Support/YAMLIOTest.cpp

@@ -1298,7 +1298,7 @@ TEST(YAMLIO, TestScalarAfterBlockScalar) {
 - block_scalac:     |
     AA
     BB
-scalar:          a
+  scalar:          a
 ...
 )");
 }