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.

llvm-svn: 17864