Add simple interface for protecting runtime functions from name

Add simple interface for protecting runtime functions from name
collisions.
 - Provide CodeGenModule::CreateRuntimeFunction which guarantees that
   the function it creates will have the provided name in the final
   module. This allows the runtime to have its functions protected
   from declarations of the same name in the source code.

 - One could argue that this is a reason to abuse the llvm::Module
   namespace for dealing with function redeclarations. However, that
   approach seems conceptually flawed to me. This one also happens to
   be somewhat more efficient.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@56899 91177308-0d34-0410-b5e6-96231b3b80d8