Subzero: Convert a few getName() methods to return a const string ref.

src/IceELFObjectWriter.cpp

Index: src/IceELFObjectWriter.cpp
diff --git a/src/IceELFObjectWriter.cpp b/src/IceELFObjectWriter.cpp
index 2d11eb3b821c3089ab79b9e9d2bff92fcfd10f67..bd0d0893dafb50c4548ea893179591c9c41f2e49 100644
--- a/src/IceELFObjectWriter.cpp
+++ b/src/IceELFObjectWriter.cpp
@@ -541,7 +541,7 @@ void ELFObjectWriter::writeAllRelocationSections(bool IsELF64) {
 void ELFObjectWriter::setUndefinedSyms(const ConstantList &UndefSyms) {
   for (const Constant *S : UndefSyms) {
     const auto Sym = llvm::cast<ConstantRelocatable>(S);
-    IceString Name = Sym->getName();
+    const IceString &Name = Sym->getName();
     assert(Sym->getOffset() == 0);
     assert(Sym->getSuppressMangling());
     SymTab->noteUndefinedSym(Name, NullSection);