Prevent alias from pointing to weak aliases.

This adds back r204781.

Original message:

Aliases are just another name for a position in a file. As such, the
regular symbol resolutions are not applied. For example, given

define void @my_func() {
  ret void
}
@my_alias = alias weak void ()* @my_func
@my_alias2 = alias void ()* @my_alias

We produce without this patch:

        .weak   my_alias
my_alias = my_func
        .globl  my_alias2
my_alias2 = my_alias

That is, in the resulting ELF file my_alias, my_func and my_alias are
just 3 names pointing to offset 0 of .text. That is *not* the
semantics of IR linking. For example, linking in a

@my_alias = alias void ()* @other_func

would require the strong my_alias to override the weak one and
my_alias2 would end up pointing to other_func.

There is no way to represent that with aliases being just another
name, so the best solution seems to be to just disallow it, converting
a miscompile into an error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204934 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/GlobalOpt/alias-resolve.ll

@@ -1,14 +1,17 @@
 ; RUN: opt < %s -globalopt -S | FileCheck %s
 
 @foo1 = alias void ()* @foo2
-; CHECK: @foo1 = alias void ()* @foo2
+; CHECK: @foo1 = alias void ()* @bar2
 
-@foo2 = alias weak void()* @bar1
-; CHECK: @foo2 = alias weak void ()* @bar2
+@foo2 = alias void()* @bar1
+; CHECK: @foo2 = alias void ()* @bar2
 
 @bar1  = alias void ()* @bar2
 ; CHECK: @bar1 = alias void ()* @bar2
 
+@weak1 = alias weak void ()* @bar2
+; CHECK: @weak1 = alias weak void ()* @bar2
+
 define void @bar2() {
   ret void
 }
@@ -17,14 +20,16 @@ define void @bar2() {
 define void @baz() {
 entry:
          call void @foo1()
-; CHECK: call void @foo2()
+; CHECK: call void @bar2()
 
          call void @foo2()
-; CHECK: call void @foo2()
+; CHECK: call void @bar2()
 
          call void @bar1()
 ; CHECK: call void @bar2()
 
+         call void @weak1()
+; CHECK: call void @weak1()
          ret void
 }