[ELF] Set `referenced` bit of Undefined created by BitcodeFile

[MaskRay]

Summary

D64136 and D65584, while fixing STB_WEAK issues and improving our
compatibility with ld.bfd, can cause another STB_WEAK problem related to
LTO:

If %tundef.o has an undefined reference on f,
and %tweakundef.o has a weak undefined reference on f,
%tdef.o has a definition of f

ld.lld %tundef.o %tweakundef.o --start-lib %tdef.o --end-lib

1. %tundef.o doesn't set referenced bit.
2. %weakundef.o changes the binding from STB_GLOBAL to STB_WEAK
3. %tdef.o is not fetched because the binding is weak.

Step (1) is incorrect. This patch sets the referenced bit of Undefined
created by bitcode files.

[ruiu]

LGTM

[MaskRay]

Seems we don't have enough tests.

Add two other cases that the lazy object should be fetched

ELF/InputFiles.cpp

static Symbol *createBitcodeSymbol(const std::vector<bool> &keptComdats,
   uint8_t visibility = mapVisibility(objSym.getVisibility());
   bool canOmitFromDynSym = objSym.canBeOmittedFromSymbolTable();
 
   int c = objSym.getComdatIndex();
   if (objSym.isUndefined() || (c != -1 && !keptComdats[c])) {
     Undefined newSym(&f, name, binding, visibility, type);
     if (canOmitFromDynSym)
       newSym.exportDynamic = false;
-    return symtab->addSymbol(newSym);
+    Symbol *ret = symtab->addSymbol(newSym);
+    ret->referenced = true;
+    return ret;
   }
 
   if (objSym.isCommon())
     return symtab->addSymbol(
         CommonSymbol{&f, name, binding, visibility, STT_OBJECT,
                      objSym.getCommonAlignment(), objSym.getCommonSize()});
 
   Defined newSym(&f, name, binding, visibility, type, 0, 0, nullptr);

test/ELF/lto/Inputs/undef.ll

+target triple = "x86_64-unknown-linux-gnu"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare void @foo()

test/ELF/lto/undef-weak-lazy.ll

+; REQUIRES: x86
+; RUN: llvm-as %S/Inputs/undef.ll -o %tundef.o
+; RUN: llvm-as %s -o %tweakundef.o
+; RUN: llvm-as %S/Inputs/archive-3.ll -o %tdef.o
+
+;; Test that the lazy bitcode %tdef.o is fetched.
+; RUN: ld.lld %tundef.o --start-lib %tdef.o --end-lib -shared -o %t.so
+; RUN: llvm-nm %t.so | FileCheck %s
+
+;; Test %tweakundef.o does not change STB_GLOBAL to STB_WEAK.
+; RUN: ld.lld %tundef.o %tweakundef.o --start-lib %tdef.o --end-lib -shared -o %t.so
+; RUN: llvm-nm %t.so | FileCheck %s
+
+;; %tweakundef.o does not fetch %tdef.o but %tundef.o does.
+; RUN: ld.lld --start-lib %tdef.o --end-lib %tweakundef.o %tundef.o -shared -o %t.so
+; RUN: llvm-nm %t.so | FileCheck %s
+
+; CHECK: T foo
+
+target triple = "x86_64-unknown-linux-gnu"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare extern_weak void @foo()