CodeGen: fix runtime function dll storage

Properly attribute DLL storage to runtime functions.  When generating the
runtime function, scan for an existing declaration which may provide an explicit
declaration (local storage) or a DLL import or export storage from the user.
Honour that if available.  Otherwise, if building with a local visibility of the
public or standard namespaces (-flto-visibility-public-std), give the symbols
local storage (it indicates a /MT[d] link, so static runtime).  Otherwise,
assume that the link is dynamic, and give the runtime function dllimport
storage.

This allows for implementations to get the correct storage as long as they are
properly declared, the user to override the import storage, and in case no
explicit storage is given, use of the import storage.

llvm-svn: 289776