This patch causes clang to reject alias attributes that point to undefined

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

void f1(void) __attribute__((alias("g1")));

This patch is implemented in CodeGen. It is quiet a bit simpler and more
compatible with gcc than implementing it in Sema. The downside is that the
errors only fire during -emit-llvm.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193161 91177308-0d34-0410-b5e6-96231b3b80d8