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/test/Bitcode/mdstring-high-bits.ll

@@ -0,0 +1,9 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; PR21882: confirm we don't crash when high bits are set in a character in a
+; metadata string.
+
+; CHECK: !name = !{!0}
+!name = !{!0}
+; CHECK: !0 = metadata !{metadata !"\80"}
+!0 = metadata !{metadata !"\80"}