The MDString class stored a StringRef to the string which was already in a

The MDString class stored a StringRef to the string which was already in a
StringMap. This was redundant and unnecessarily bloated the MDString class.

Because the MDString class is a "Value" and will never have a "name", and
because the Name field in the Value class is a pointer to a StringMap entry, we
repurpose the Name field for an MDString. It stores the StringMap entry in the
Name field, and uses the normal methods to get the string (name) back.

PR12474

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154429 91177308-0d34-0410-b5e6-96231b3b80d8