Add support for embedded metadata to LLVM. This introduces two new types of

Add support for embedded metadata to LLVM. This introduces two new types of
Constant, MDString and MDNode which can only be used by globals with a name
that starts with "llvm." or as arguments to a function with the same naming
restriction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68420 91177308-0d34-0410-b5e6-96231b3b80d8