[ELF] Mark symbols referenced from linker script as live

[phosek]

Summary

This is necessary to ensure that sections containing symbols referenced
from linker scripts (e.g. in data commands) don't get GC'd.

[ruiu]

LGTM

test/ELF/linkerscript/data-commands-gc.s

3–9

Can you write this in one line?

ELF/MarkLive.cpp

template <class ELFT> void elf::markLive() {
   };
 
   // Add GC root symbols.
   MarkSymbol(Symtab<ELFT>::X->find(Config->Entry));
   MarkSymbol(Symtab<ELFT>::X->find(Config->Init));
   MarkSymbol(Symtab<ELFT>::X->find(Config->Fini));
   for (StringRef S : Config->Undefined)
     MarkSymbol(Symtab<ELFT>::X->find(S));
+  for (StringRef S : Script->Opt.ReferencedSymbols)
+    MarkSymbol(Symtab<ELFT>::X->find(S));
 
   // Preserve externally-visible symbols if the symbols defined by this
   // file can interrupt other ELF file's symbols at runtime.
   for (const Symbol *S : Symtab<ELFT>::X->getSymbols())
     if (S->includeInDynsym())
       MarkSymbol(S->body());
 
   // Preserve special sections and those which are specified in linker

test/ELF/linkerscript/data-commands-gc.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS                \
+# RUN:  {                            \
+# RUN:    .text : {                  \
+# RUN:      *(.text*)                \
+# RUN:      QUAD(bar)                \
+# RUN:    }                          \
+# RUN:  }" > %t.script
+# RUN: ld.lld --gc-sections -o %t %t.o --script %t.script | FileCheck -allow-empty %s
+
+# CHECK-NOT: unable to evaluate expression: input section .rodata.bar has no output section assigned
+
+.section .rodata.bar
+.quad 0x1122334455667788
+.global bar
+bar:
+
+.section .text
+.global _start
+_start:
+  nop