[LTO][Legacy] Use the same defaults as non-legacy LTO

[glandium]

Summary

The main motivation is that the transition to opaque pointers relies on
LTOLLVMContext setting the default from lto::Config, but legacy LTO is
not using that and that can cause problems linking objects produced by
clang that does use opaque pointers by default.

It seems to make sense for defaults to align between legacy and
non-legacy LTO more generally.

Fixes #57734

[steven_wu]

Looks like this break lots of tests which all needs to be updated. Need to look at what other defaults are changed.

Also please summarize the change and add the motivation/reason to the commit message.

[glandium]

Added a null check.

llvm/include/llvm/LTO/Config.h

 };
 /// A derived class of LLVMContext that initializes itself according to a given
 /// Config object. The purpose of this class is to tie ownership of the
 /// diagnostic handler to the context, as opposed to the Config object (which
 /// may be ephemeral).
 // FIXME: This should not be required as diagnostic handler is not callback.
 struct LTOLLVMContext : LLVMContext {
 
+  LTOLLVMContext() : LTOLLVMContext(Config()) {}
+
   LTOLLVMContext(const Config &C) : DiagHandler(C.DiagHandler) {
     setDiscardValueNames(C.ShouldDiscardValueNames);
     enableDebugTypeODRUniquing();
+    if (DiagHandler)
       setDiagnosticHandler(
           std::make_unique<LTOLLVMDiagnosticHandler>(&DiagHandler), true);
     if (!hasSetOpaquePointersValue())
       setOpaquePointers(C.OpaquePointers);
   }
   DiagnosticHandlerFunction DiagHandler;
 };
 
 }
 }
 
 #endif

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

 #include "llvm/Analysis/ProfileSummaryInfo.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
 #include "llvm/Bitcode/BitcodeWriterPass.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DiagnosticPrinter.h"
-#include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/LLVMRemarkStreamer.h"
+#include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/Mangler.h"
 #include "llvm/IR/PassTimingInfo.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/IRReader/IRReader.h"
+#include "llvm/LTO/Config.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/LTO/SummaryBasedOptimizations.h"
 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Object/IRObjectFile.h"
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Passes/StandardInstrumentations.h"
 #include "llvm/Remarks/HotnessThresholdParser.h"

 
 #if !defined(_MSC_VER) && !defined(__MINGW32__)
 #include <unistd.h>
 #else
 #include <io.h>
 #endif
 
 using namespace llvm;
+using namespace lto;
 
 #define DEBUG_TYPE "thinlto"
 
 namespace llvm {
 // Flags -discard-value-names, defined in LTOCodeGenerator.cpp
 extern cl::opt<bool> LTODiscardValueNames;
 extern cl::opt<std::string> RemarksFilename;
 extern cl::opt<std::string> RemarksPasses;
for (auto IndexCount : ModulesOrdering) {
               ProducedBinaries[count] = std::move(ErrOrBuffer.get());
             else
               ProducedBinaryFiles[count] = writeGeneratedObject(
                   count, CacheEntryPath, *ErrOrBuffer.get());
             return;
           }
         }
 
-        LLVMContext Context;
+        LTOLLVMContext Context;
         Context.setDiscardValueNames(LTODiscardValueNames);
         Context.enableDebugTypeODRUniquing();
         auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
             Context, RemarksFilename, RemarksPasses, RemarksFormat,
             RemarksWithHotness, RemarksHotnessThreshold, count);
         if (!DiagFileOrErr) {
           errs() << "Error: " << toString(DiagFileOrErr.takeError()) << "\n";
           report_fatal_error("ThinLTO: Can't get an output file for the "