[DebugInfo] Only call upgradeCULocals() at module level (#68965)

Loading a 2GB bitcode file, I noticed that we spend minutes just running
upgradeCULocals(). Apparently it gets invoked every time a metadata
block is loaded, which will be once at the module level and then once
per function. However, the relevant metadata only exists at the module
level, so running this upgrade per function is unnecessary.

Author: nikic

llvm/lib/Bitcode/Reader/MetadataLoader.cpp

@@ -705,10 +705,11 @@ class MetadataLoader::MetadataLoaderImpl {
     return Error::success();
   }
 
-  void upgradeDebugInfo() {
+  void upgradeDebugInfo(bool ModuleLevel) {
     upgradeCUSubprograms();
     upgradeCUVariables();
-    upgradeCULocals();
+    if (ModuleLevel)
+      upgradeCULocals();
   }
 
   void callMDTypeCallback(Metadata **Val, unsigned TypeID);
@@ -1085,7 +1086,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
       // Reading the named metadata created forward references and/or
       // placeholders, that we flush here.
       resolveForwardRefsAndPlaceholders(Placeholders);
-      upgradeDebugInfo();
+      upgradeDebugInfo(ModuleLevel);
       // Return at the beginning of the block, since it is easy to skip it
       // entirely from there.
       Stream.ReadBlockEnd(); // Pop the abbrev block context.
@@ -1116,7 +1117,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
       resolveForwardRefsAndPlaceholders(Placeholders);
-      upgradeDebugInfo();
+      upgradeDebugInfo(ModuleLevel);
       return Error::success();
     case BitstreamEntry::Record:
       // The interesting case.