[C++20][Modules][6/8] Record direct module imports.

This is a small cache to avoid having to check both Exports and
Imports.

Differential Revision: https://reviews.llvm.org/D118589

clang/include/clang/Sema/Sema.h

@@ -2219,6 +2219,9 @@ class Sema final {
   /// The global module fragment of the current translation unit.
   clang::Module *GlobalModuleFragment = nullptr;
 
+  /// The modules we imported directly.
+  llvm::SmallPtrSet<clang::Module *, 8> DirectModuleImports;
+
   /// Namespace definitions that we will export when they finish.
   llvm::SmallPtrSet<const NamespaceDecl*, 8> DeferredExportedNamespaces;
 
@@ -2246,6 +2249,10 @@ class Sema final {
     return Entity->getOwningModule();
   }
 
+  bool isModuleDirectlyImported(const Module *M) {
+    return DirectModuleImports.contains(M);
+  }
+
   /// Make a merged definition of an existing hidden definition \p ND
   /// visible at the specified location.
   void makeMergedDefinitionVisible(NamedDecl *ND);

clang/lib/Sema/SemaModule.cpp

@@ -514,6 +514,11 @@ DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
     assert(ThisModule && "was expecting a module if building one");
   }
 
+  // In some cases we need to know if an entity was present in a directly-
+  // imported module (as opposed to a transitive import).  This avoids
+  // searching both Imports and Exports.
+  DirectModuleImports.insert(Mod);
+
   return Import;
 }