YAML: Null terminate block scalar's value.

The commit null terminates the string value in the `yaml::BlockScalarNode`
class.

This change is motivated by the initial MIR serialization commit (r237708)
that I reverted in r237730 because the LLVM IR source from the block
scalar node wasn't terminated by a null character and thus the buildbots
failed on one testcase sometimes. This change enables me to recommit 
the reverted commit. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237942 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/YAMLParser.cpp

@@ -2379,7 +2379,8 @@ parse_property:
                 , T.Range);
   case Token::TK_BlockScalar: {
     getNext();
-    StringRef StrCopy = StringRef(T.Value).copy(NodeAllocator);
+    StringRef NullTerminatedStr(T.Value.c_str(), T.Value.length() + 1);
+    StringRef StrCopy = NullTerminatedStr.copy(NodeAllocator).drop_back();
     return new (NodeAllocator)
         BlockScalarNode(stream.CurrentDoc, AnchorInfo.Range.substr(1),
                         TagInfo.Range, StrCopy, T.Range);