Now that we have ghost linkage, we can force resolution of external symbols

Now that we have ghost linkage, we can force resolution of external symbols
immediately instead of lazily.

In this program, for example:

int main() {
  printf("hello world\n");
  printf("hello world\n");
  printf("hello world\n");
  printf("hello world\n");
}

We used to have to go through compilation callback 4 times (once for each
call to printf), now we don't go to it at all.

Thanks to Misha for noticing this, and for adding the initial ghost linkage
patches.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17864 91177308-0d34-0410-b5e6-96231b3b80d8