`MDString`s can have arbitrary characters in them. Prevent an assertion
that fired in `BitcodeWriter` because of sign extension by copying the
characters into the record as `unsigned char`s.
Based on a patch by Keno Fischer; fixes PR21882.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224077 91177308-0d34-0410-b5e6-96231b3b80d8