[clang] Use *Map::try_emplace (NFC) (#143563)

- try_emplace(Key) is shorter than insert({Key, nullptr}).
- try_emplace performs value initialization without value parameters.
- We overwrite values on successful insertion anyway.

clang/include/clang/Basic/IdentifierTable.h

@@ -731,7 +731,7 @@ public:
   /// introduce or modify an identifier. If they called get(), they would
   /// likely end up in a recursion.
   IdentifierInfo &getOwn(StringRef Name) {
-    auto &Entry = *HashTable.insert(std::make_pair(Name, nullptr)).first;
+    auto &Entry = *HashTable.try_emplace(Name).first;
 
     IdentifierInfo *&II = Entry.second;
     if (II)

clang/include/clang/ExtractAPI/API.h

@@ -1499,7 +1499,7 @@ APISet::createRecord(StringRef USR, StringRef Name,
                      CtorArgsContTy &&...CtorArgs) {
   // Ensure USR refers to a String stored in the allocator.
   auto USRString = copyString(USR);
-  auto Result = USRBasedLookupTable.insert({USRString, nullptr});
+  auto Result = USRBasedLookupTable.try_emplace(USRString);
   RecordTy *Record;
 
   // Create the record if it does not already exist

clang/lib/CodeGen/CodeGenTypes.cpp

@@ -726,7 +726,7 @@ llvm::Type *CodeGenTypes::ConvertType(QualType T) {
     auto *MPTy = cast<MemberPointerType>(Ty);
     if (!getCXXABI().isMemberPointerConvertible(MPTy)) {
       auto *C = MPTy->getMostRecentCXXRecordDecl()->getTypeForDecl();
-      auto Insertion = RecordsWithOpaqueMemberPointers.insert({C, nullptr});
+      auto Insertion = RecordsWithOpaqueMemberPointers.try_emplace(C);
       if (Insertion.second)
         Insertion.first->second = llvm::StructType::create(getLLVMContext());
       ResultType = Insertion.first->second;

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

@@ -173,7 +173,7 @@ DependencyScanningFilesystemSharedCache::CacheShard::getOrEmplaceEntryForUID(
     llvm::sys::fs::UniqueID UID, llvm::vfs::Status Stat,
     std::unique_ptr<llvm::MemoryBuffer> Contents) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
-  auto [It, Inserted] = EntriesByUID.insert({UID, nullptr});
+  auto [It, Inserted] = EntriesByUID.try_emplace(UID);
   auto &CachedEntry = It->getSecond();
   if (Inserted) {
     CachedFileContents *StoredContents = nullptr;