[C++20] [Modules] Implement diagnose for exposured partially

Tracked at llvm#112294

This patch implements from [basic.link]p14 to [basic.link]p18 partially.

The explicitly missing parts are:
- Anything related to specializations.
- Decide if a pointer is associated with a TU-local value at compile
  time.
- [basic.link]p15.1.2 to decide if a type is TU-local.
- Diagnose if TU-local functions from other TU are collected to the
  overload set. See [basic.link]p19, the call to 'h(N::A{});' in
  translation unit #2

There should be other implicitly missing parts as the wording uses
"names" briefly several times. But to implement this precisely, we have
to visit the whole AST, including Decls, Expression and Types, which may
be harder to implement and be more time-consuming for compilation time.
So I choose to implement the common parts.

It won't be too bad to miss some cases since we DIDN'T do any such
checks in the past 3 years. Any new check is an improvement. Given
modules have been basically available since clang15 without such checks,
it will be user unfriendly if we give a hard error now. And there are
a lot of cases which violating the rule actually just fine. So I decide
to emit it as warnings instead of hard errors.

Author: ChuanqiXu9

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12391,6 +12391,13 @@ def err_invalid_module_name : Error<"%0 is an invalid name for a module">;
 def err_extern_def_in_header_unit : Error<
   "non-inline external definitions are not permitted in C++ header units">;
 
+def warn_exposure : Warning <
+  "TU local entity %0 is exposed">,
+  InGroup<DiagGroup<"TU-local-entity-exposure">>;
+def warn_reference_tu_local_entity_in_other_tu : Warning <
+  "instantiation of %0 triggers reference to TU-local entity %1 from other TU '%2'">,
+  InGroup<DiagGroup<"reference-tu-local-entity-in-other-tu">>;
+
 def warn_experimental_header_unit : Warning<
   "the implementation of header units is in an experimental phase">,
   InGroup<DiagGroup<"experimental-header-units">>;

clang/include/clang/Sema/Sema.h

@@ -9945,6 +9945,20 @@ class Sema final : public SemaBase {
 
   VisibleModuleSet VisibleModules;
 
+  /// Whether we had imported any named modules.
+  bool HadImportedNamedModules = false;
+  /// The set of instantiations we need to check if they references TU-local
+  /// entity from TUs. This only makes sense if we imported any named modules.
+  llvm::SmallVector<std::pair<FunctionDecl *, SourceLocation>>
+      PendingCheckReferenceForTULocal;
+  /// Implement [basic.link]p18, which requires that we can't use TU-local
+  /// entities from other TUs (ignoring header units).
+  void checkReferenceToTULocalFromOtherTU(FunctionDecl *FD,
+                                          SourceLocation PointOfInstantiation);
+  /// Implement [basic.link]p17, which diagnose for non TU local exposure in
+  /// module interface or module partition.
+  void checkExposure(const TranslationUnitDecl *TU);
+
   ///@}
 
   //

clang/lib/Sema/Sema.cpp

@@ -1616,6 +1616,8 @@ void Sema::ActOnEndOfTranslationUnit() {
 
   if (!PP.isIncrementalProcessingEnabled())
     TUScope = nullptr;
+
+  checkExposure(Context.getTranslationUnitDecl());
 }