[ObjectYAML] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/lib/ObjectYAML/DWARFEmitter.cpp

@@ -96,12 +96,11 @@ Error DWARFYAML::emitDebugStr(raw_ostream &OS, const DWARFYAML::Data &DI) {
 StringRef DWARFYAML::Data::getAbbrevTableContentByIndex(uint64_t Index) const {
   assert(Index < DebugAbbrev.size() &&
          "Index should be less than the size of DebugAbbrev array");
-  auto It = AbbrevTableContents.find(Index);
-  if (It != AbbrevTableContents.cend())
+  auto [It, Inserted] = AbbrevTableContents.try_emplace(Index);
+  if (!Inserted)
     return It->second;
 
-  std::string AbbrevTableBuffer;
-  raw_string_ostream OS(AbbrevTableBuffer);
+  raw_string_ostream OS(It->second);
 
   uint64_t AbbrevCode = 0;
   for (const DWARFYAML::Abbrev &AbbrevDecl : DebugAbbrev[Index].Table) {
@@ -123,9 +122,7 @@ StringRef DWARFYAML::Data::getAbbrevTableContentByIndex(uint64_t Index) const {
   // consisting of a 0 byte for the abbreviation code.
   OS.write_zeros(1);
 
-  AbbrevTableContents.insert({Index, AbbrevTableBuffer});
-
-  return AbbrevTableContents[Index];
+  return It->second;
 }
 
 Error DWARFYAML::emitDebugAbbrev(raw_ostream &OS, const DWARFYAML::Data &DI) {