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. 

llvm-svn: 237942