[CrossTU] Avoid repeated hash lookups (NFC) (#126380)

Author: kazutakahirata

Diff for: clang/lib/CrossTU/CrossTranslationUnit.cpp

@@ -453,15 +453,16 @@ CrossTranslationUnitContext::ASTUnitStorage::getASTUnitForFunction(
       return std::move(IndexLoadError);
 
     // Check if there is an entry in the index for the function.
-    if (!NameFileMap.count(FunctionName)) {
+    auto It = NameFileMap.find(FunctionName);
+    if (It == NameFileMap.end()) {
       ++NumNotInOtherTU;
       return llvm::make_error<IndexError>(index_error_code::missing_definition);
     }
 
     // Search in the index for the filename where the definition of FunctionName
     // resides.
     if (llvm::Expected<ASTUnit *> FoundForFile =
-            getASTUnitForFile(NameFileMap[FunctionName], DisplayCTUProgress)) {
+            getASTUnitForFile(It->second, DisplayCTUProgress)) {
 
       // Update the cache.
       NameASTUnitMap[FunctionName] = *FoundForFile;