Define a function with "... llvm::func...".

Define a function with "... llvm::func...".

Using this instead of
namespace llvm {
  func...
}

Has the advantage that the build fails with a compiler error if it gets out
of sync with the .h file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234515 91177308-0d34-0410-b5e6-96231b3b80d8