Add undefined symbol test

Author: mysterymath

lld/ELF/MarkLive.cpp

@@ -248,7 +248,6 @@ template <class ELFT> void MarkLive<ELFT>::printWhyLive(Symbol *s) const {
   // If the symbol isn't live, return.
   if (!whyLive.contains(s)) {
     auto *d = dyn_cast<Defined>(s);
-    // TODO: Test
     if (!d)
       return;
     // TODO: Test both cases

lld/test/ELF/why-live.s

@@ -42,3 +42,7 @@ jmp test_from_unsized
 .section .test_dead,"ax",@progbits
 test_dead:
 jmp test_dead
+
+## Undefined symbols are not considered live.
+# RUN: ld.lld %t.o -o /dev/null --gc-sections --why-live=test_undef -u test_undef | count 0
+