[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Richard Smith via cfe-commits]

================
@@ -1403,94 +1421,146 @@ bool HeaderSearch::isFileMultipleIncludeGuarded(FileEntryRef File) const {
 void HeaderSearch::MarkFileModuleHeader(FileEntryRef FE,
                                         ModuleMap::ModuleHeaderRole Role,
                                         bool isCompilingModuleHeader) {
-  bool isModularHeader = ModuleMap::isModular(Role);
-
   // Don't mark the file info as non-external if there's nothing to change.
   if (!isCompilingModuleHeader) {
-    if (!isModularHeader)
+    if ((Role & ModuleMap::ExcludedHeader))
       return;
     auto *HFI = getExistingFileInfo(FE);
     if (HFI && HFI->isModuleHeader)
       return;
   }
 
   auto &HFI = getFileInfo(FE);
-  HFI.isModuleHeader |= isModularHeader;
+  HFI.mergeModuleMembership(Role);
   HFI.isCompilingModuleHeader |= isCompilingModuleHeader;
----------------
zygoloid wrote:

It looks to me like we're now calling `getFileInfo(FE)` even in cases where the info doesn't change, for a textual header. I think that's what's causing the regression we're seeing -- we're now considering a repeatedly-used module map file to be affecting, even though it isn't, because we end up with a local `HeaderFileInfo` instead of an imported one.

https://github.com/llvm/llvm-project/pull/83660