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