Revert "[DebugInfo][RemoveDIs] Convert debug-info modes when loading bitcode (#78967)"

This reverts commit 215b8f1e25.

Numerous builders exploded from this X_X, for example

  https://lab.llvm.org/buildbot/#/builders/46/builds/62657

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -100,9 +100,6 @@ static cl::opt<bool> ExpandConstantExprs(
     cl::desc(
         "Expand constant expressions to instructions for testing purposes"));
 
-// Declare external flag for whether we're using the new debug-info format.
-extern llvm::cl::opt<bool> UseNewDbgInfoFormat;
-
 namespace {
 
 enum {
@@ -6632,9 +6629,6 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
   if (Error Err = materializeMetadata())
     return Err;
 
-  bool NewDebugInfoRequested = F->IsNewDbgInfoFormat;
-  F->IsNewDbgInfoFormat = false;
-
   // Move the bit stream to the saved position of the deferred function body.
   if (Error JumpFailed = Stream.JumpToBit(DFII->second))
     return JumpFailed;
@@ -6710,14 +6704,6 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
   // Look for functions that rely on old function attribute behavior.
   UpgradeFunctionAttributes(*F);
 
-  // If we've materialized a function set up in "new" debug-info mode, the
-  // contents just loaded will still be in dbg.value mode. Switch to the new
-  // mode now. NB: we can add more complicated logic here in the future to
-  // correctly identify when we do and don't need to autoupgrade.
-  if (NewDebugInfoRequested) {
-    F->convertToNewDbgValues();
-  }
-
   // Bring in any functions that this function forward-referenced via
   // blockaddresses.
   return materializeForwardReferencedFunctions();
@@ -8041,15 +8027,6 @@ BitcodeModule::getModuleImpl(LLVMContext &Context, bool MaterializeAll,
     if (Error Err = R->materializeForwardReferencedFunctions())
       return std::move(Err);
   }
-
-  // If we are operating in a "new debug-info" context, upgrade the debug-info
-  // in the loaded module. This is a transitional approach as we enable "new"
-  // debug-info in LLVM, which will eventually be pushed down into the
-  // autoupgrade path once the bitcode-encoding is finalised. Non-materialised
-  // functions will be upgraded in the materialize method.
-  if (UseNewDbgInfoFormat && !M->IsNewDbgInfoFormat)
-    M->convertToNewDbgValues();
-
   return std::move(M);
 }