Revert "This patch causes clang to reject alias attributes that point to undefined...

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