Add test for interaction of --gc-sections and undefined references

It is possible for a link to fail with an undefined reference, unless
--gc-sections is specified, removing the reference in the process. This
doesn't look to be tested anywhere explicitly, so I thought it useful
to add a test for it to ensure the behaviour is maintained.

Reviewers: ruiu

Differential Revision: https://reviews.llvm.org/D42299

llvm-svn: 323099

lld/test/ELF/gc-sections-no-undef-error.s

@@ -0,0 +1,19 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+
+# Sanity check that the link will fail with the undefined error without
+# gc-sections.
+# RUN: not ld.lld %t.o -o %t 2>&1 | FileCheck %s
+# CHECK: error: undefined symbol: undefined
+
+# RUN: ld.lld %t.o --gc-sections -o %t
+
+.section .text.unused,"ax",@progbits
+unused:
+  callq undefined
+
+.text
+.global _start
+_start:
+  nop