Treat aliases as definitions.

This fixes pr17639.

Before this patch clang would consider

void foo(void) __attribute((alias("__foo")));

a declaration. It now correctly handles it as a definition.

Initial patch by Alp Toker. I added support for variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193200 91177308-0d34-0410-b5e6-96231b3b80d8