[patch][pr17535] Reject alias attributes that point to undefined name (V2).

[Rafael Espíndola]

The attached 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")));

Unlike the previous patch this one is implemented in CodeGen. It is
quiet a bit simpler. The downside is that the errors only fire during
-emit-llvm.

Cheers,
Rafael
-------------- next part --------------
A non-text attachment was scrubbed...
Name: t.patch
Type: application/octet-stream
Size: 6907 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20131021/3291944a/attachment.obj>