Bitcode: Use unsigned char to record MDStrings

`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.

llvm-svn: 224077

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -800,7 +800,7 @@ static void WriteModuleMetadata(const Module *M,
       }
 
       // Code: [strchar x N]
-      Record.append(MDS->begin(), MDS->end());
+      Record.append(MDS->bytes_begin(), MDS->bytes_end());
 
       // Emit the finished record.
       Stream.EmitRecord(bitc::METADATA_STRING, Record, MDSAbbrev);