Document an interesting LTO case.

This is a reduction from the only symbol in a build of clang that the
gold plugin internalizes but lld does not.

llvm-svn: 272845

lld/test/ELF/lto/Inputs/unnamed-addr-lib.s

@@ -0,0 +1,6 @@
+        .protected foo
+        .global foo
+foo:
+
+        .global bar
+bar:

lld/test/ELF/lto/unnamed-addr-lib.ll

@@ -0,0 +1,21 @@
+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-mc %p/Inputs/unnamed-addr-lib.s -o %t2.o -filetype=obj -triple=x86_64-pc-linux
+; RUN: ld.lld %t2.o -shared -o %t2.so
+; RUN: ld.lld -m elf_x86_64 %t.o %t2.so -o %t.so -save-temps -shared
+; RUN: llvm-dis %t.so.lto.bc -o - | FileCheck %s
+
+; This documents a small limitation of lld's internalization logic. We decide
+; that bar should be in the symbol table because if it is it will preempt the
+; one in the shared library.
+; We could add one extra bit for ODR so that we know that preemption is not
+; necessary, but that is probably not worth it.
+
+; CHECK: @foo = internal unnamed_addr constant i8 42
+; CHECK: @bar = weak_odr unnamed_addr constant i8 42
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@foo = linkonce_odr unnamed_addr constant i8 42
+@bar = linkonce_odr unnamed_addr constant i8 42