[clang][deps] Skip writing DIAG_PRAGMA_MAPPINGS record (llvm#70874)

Following up on llvm#69975, this patch skips writing `DIAG_PRAGMA_MAPPINGS`
as well. Deserialization of this PCM record is still showing up in
profiles, since it needs to be VBR-decoded for every transitively loaded
PCM file.

The scanner doesn't make any guarantees about diagnostic accuracy (and
it even disables all warnings), so skipping this record should be safe.

(cherry picked from commit 22c6851)

Author: jansvoboda11

clang/include/clang/Lex/HeaderSearchOptions.h

@@ -224,6 +224,10 @@ class HeaderSearchOptions {
   /// Primarily used to speed up deserialization during dependency scanning.
   unsigned ModulesSkipHeaderSearchPaths : 1;
 
+  /// Whether to entirely skip writing pragma diagnostic mappings.
+  /// Primarily used to speed up deserialization during dependency scanning.
+  unsigned ModulesSkipPragmaDiagnosticMappings : 1;
+
   unsigned ModulesHashContent : 1;
 
   /// Whether we should include all things that could impact the module in the
@@ -244,7 +248,8 @@ class HeaderSearchOptions {
         ValidateASTInputFilesContent(false), UseDebugInfo(false),
         ModulesValidateDiagnosticOptions(true),
         ModulesSkipDiagnosticOptions(false),
-        ModulesSkipHeaderSearchPaths(false), ModulesHashContent(false),
+        ModulesSkipHeaderSearchPaths(false),
+        ModulesSkipPragmaDiagnosticMappings(false), ModulesHashContent(false),
         ModulesStrictContextHash(false) {}
 
   /// AddPath - Add the \p Path path to the specified \p Group list.

clang/lib/Serialization/ASTWriter.cpp

@@ -1267,8 +1267,8 @@ void ASTWriter::writeUnhashedControlBlock(Preprocessor &PP,
   AddString(FSOpts.WorkingDir, Record);
   Stream.EmitRecord(FILE_SYSTEM_OPTIONS, Record);
 
-  // Write out the diagnostic/pragma mappings.
-  WritePragmaDiagnosticMappings(Diags, /* isModule = */ WritingModule);
+  if (!HSOpts.ModulesSkipPragmaDiagnosticMappings)
+    WritePragmaDiagnosticMappings(Diags, /* isModule = */ WritingModule);
 
   // Header search entry usage.
   auto HSEntryUsage = PP.getHeaderSearchInfo().computeUserEntryUsage();

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -436,6 +436,8 @@ class DependencyScanningAction : public tooling::ToolAction {
     ScanInstance.getHeaderSearchOpts().ModulesStrictContextHash = true;
     ScanInstance.getHeaderSearchOpts().ModulesSkipDiagnosticOptions = true;
     ScanInstance.getHeaderSearchOpts().ModulesSkipHeaderSearchPaths = true;
+    ScanInstance.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings =
+        true;
 
     // Avoid some checks and module map parsing when loading PCM files.
     ScanInstance.getPreprocessorOpts().ModulesCheckRelocated = false;