Initial update to VMCore to use Twines for string arguments.

Initial update to VMCore to use Twines for string arguments.
 - The only meat here is in Value.{h,cpp} the rest is essential 'const
   std::string &' -> 'const Twine &'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77048 91177308-0d34-0410-b5e6-96231b3b80d8