[clang] [clang][modulemap] Lazily load module maps by header name (PR #181916)

[Michael Spencer via cfe-commits]

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@@ -6316,6 +6316,15 @@ llvm::Error ASTReader::ReadSubmoduleBlock(ModuleFile &F,
       CurrentModule->ConfigMacrosExhaustive = ConfigMacrosExhaustive;
       CurrentModule->ModuleMapIsPrivate = ModuleMapIsPrivate;
       CurrentModule->NamedModuleHasInit = NamedModuleHasInit;
+
+      if (!ParentModule && !F.BaseDirectory.empty()) {
+        if (auto Dir = FileMgr.getOptionalDirectoryRef(F.BaseDirectory))
+          CurrentModule->Directory = *Dir;
+      } else if (ParentModule && ParentModule->Directory) {
+        // Submodules inherit the directory from their parent.
+        CurrentModule->Directory = ParentModule->Directory;
+      }
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Bigcheese wrote:

This only causes problems when lazy loading is enabled, otherwise we load PCMs/module maps in a different order that doesn't expose this issue. When loading the module map first it correctly sets the directory.

Maybe this can be reproduced with an explicit build, `-fmodule-file=` and `
#pragma clang __debug module_map`? I'll see if that works.

https://github.com/llvm/llvm-project/pull/181916