[ELF] - Add a test for --gc-sections --undefined=foo combination.

[grimar]

Summary

GC code contains following logic uncovered by tests:
(nothing fails if I remove these 2 lines)

for (StringRef S : Config->Undefined)
  MarkSymbol(Symtab->find(S));

As far I can tell we never had test for that before,
patch fixes it.

[ruiu]

It looks like this patch contains only a test. Where is the patch to fix the issue?

[grimar]

In D36519#836623, @ruiu wrote:

It looks like this patch contains only a test. Where is the patch to fix the issue?

There is no issue. We just missing the test.

I tried to remove following lines:

for (StringRef S : Config->Undefined)
  MarkSymbol(Symtab->find(S));

from elf::markLive() and all our tests passed.
So desided to add this testcase.

[ruiu]

LGTM

test/ELF/gc-sections-undefined.s

3

foo and bar are testing the same thing, so remove bar.

test/ELF/gc-sections-undefined.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t1 --gc-sections --undefined=foo --undefined=bar
+# RUN: llvm-readobj -t %t1 | FileCheck %s
+
+# CHECK: bar
+# CHECK: foo
+
+.section .foo,"ax"
+.global foo
+foo:
+
+.section .bar,"ax"
+.global bar
+bar:
+
+.section .text,"ax"
+.globl _start
+_start:
+ .quad foo