Revert "This patch causes clang to reject alias attributes that point to undefined names. For example, with this patch we now reject"

This reverts commit r193161.

It broke

void foo() __attribute__((alias("bar")));
void bar() {}
void zed() __attribute__((alias("foo")));

Looks like we have to fix pr17639 first :-(

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193162 91177308-0d34-0410-b5e6-96231b3b80d8