MC: Don't assume incoming StringRef's are null terminated.

This can happen when processing command line arguments, which
are often stored as std::string's and later turned into
StringRef's via std::string::data().  Unfortunately this
is not guaranteed to return a null-terminated string
until C++11, causing breakage on platforms that don't do this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192558 91177308-0d34-0410-b5e6-96231b3b80d8