Add a fixed version of r195470 back.

The fix is simply to use CurI instead of I when handling aliases to
avoid accessing a invalid iterator.

original message:

Convert linkonce* to weak* instead of strong.

Also refactor the logic into a helper function. This is an important improve
on mingw where the linker complains about mixed weak and strong symbols.
Converting to weak ensures that the symbol is not dropped, but keeps in a
comdat, making the linker happy.

llvm-svn: 195477