[ELF] - Added testcase for testing visibility of weak_odr symbols.

[grimar]

Summary

This is relative to "Bug 36166 - tools/gold/X86/comdat.ll is failing only on Debian Unstable" (PR36166).
Something changed in newer versions of gold and now together with gold-plugin
there is an issue shown in PR, symbol may get wrong visibility.

LLD works fine, but we have no testcase for the same use case, and I
suggest to add it just in case to explicitly document the behavior.

test/ELF/lto/Inputs/weakodr-visibility.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define weak_odr protected i32 @foo(i8* %this) {
+  ret i32 42
+}

test/ELF/lto/weakodr-visibility.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t1.o
+; RUN: llvm-as %p/Inputs/weakodr-visibility.ll -o %t2.o
+
+; Testcase checks we keep desired visibility of weak 
+; symbol in a library even if select different definition.
+; We change the order of input files in command line and
+; check that linker selects different symbol definitions,
+; but keeps `protected` visibility.
+
+; RUN: ld.lld %t1.o %t2.o -o %t.so -shared
+; RUN: llvm-readobj -t %t.so | FileCheck %s
+; RUN: llvm-objdump -d %t.so | FileCheck %s --check-prefix=FIRST
+; CHECK:       Symbol {
+; CHECK:        Name: foo
+; CHECK-NEXT:   Value:
+; CHECK-NEXT:   Size:
+; CHECK-NEXT:   Binding: Weak
+; CHECK-NEXT:   Type: Function
+; CHECK-NEXT:   Other [
+; CHECK-NEXT:     STV_PROTECTED
+; CHECK-NEXT:   ]
+; CHECK-NEXT:   Section:
+; CHECK-NEXT: }
+; FIRST:      foo:
+; FIRST-NEXT:   movl    $41, %eax
+
+; Now swap the files order.
+; RUN: ld.lld %t2.o %t1.o -o %t.so -shared
+; RUN: llvm-readobj -t %t.so | FileCheck %s
+; RUN: llvm-objdump -d %t.so | FileCheck %s --check-prefix=SECOND
+; SECOND:      foo:
+; SECOND-NEXT:   movl    $42, %eax
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define weak_odr i32 @foo(i8* %this) {
+  ret i32 41
+}