AsmParser: Don't crash if a null byte is inside a quoted string

We don't allow Value* to have names which contain null bytes.  The
AsmParser should reject .ll files that try to do this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223869 91177308-0d34-0410-b5e6-96231b3b80d8