Add support for handling ifuncs to GlobalValue::getBaseObject

[tejohnson]

Summary

All GlobalIndirectSymbol types (not just GlobalAlias) should return
their base object.

Without this patch LTO would warn "Unable to determine comdat of
alias!" for an ifunc.

[tejohnson]

Update per review

lib/Object/IRSymtab.cpp

Error Builder::addSymbol(const ModuleSymbolTable &Msymtab,
   Sym.Flags |= unsigned(GV->getVisibility()) << storage::Symbol::FB_visibility;
 
   if (Flags & object::BasicSymbolRef::SF_Common) {
     Uncommon().CommonSize = GV->getParent()->getDataLayout().getTypeAllocSize(
         GV->getType()->getElementType());
     Uncommon().CommonAlign = GV->getAlignment();
   }
 
-  const GlobalObject *Base = GV->getBaseObject();
-  if (!Base)
-    return make_error<StringError>("Unable to determine comdat of alias!",
-                                   inconvertibleErrorCode());
-  if (const Comdat *C = Base->getComdat()) {
+  if (const Comdat *C = GV->getComdat()) {
     auto P = ComdatMap.insert(std::make_pair(C, Comdats.size()));
     Sym.ComdatIndex = P.first->second;
 
     if (P.second) {
       storage::Comdat Comdat;
       setStr(Comdat.Name, C->getName());
       Comdats.push_back(Comdat);
     }

test/LTO/Resolution/X86/ifunc.ll

+; RUN: opt -module-summary -o %t.bc %s
+; RUN: llvm-lto2 run %t.bc -r %t.bc,foo,pl -o %t2
+; RUN: llvm-nm %t2.0 | FileCheck %s
+; CHECK: T foo
+; CHECK: t foo_ifunc
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@foo = ifunc i32 (i32), i64 ()* @foo_ifunc
+
+define internal i64 @foo_ifunc() {
+entry:
+  ret i64 0
+}