[C++20] [Modules] Support to export declarations in language linkage

Close https://github.com/llvm/llvm-project/issues/60405

See the discussion in the above link for the background.

What the patch does:
- Rename `Module::ModuleKind::GlobalModuleFragment` to
  `Module::ModuleKind::ExplicitGlobalModuleFragment`.
- Add another module kind `ImplicitGlobalModuleFragment` to
  `ModuleKind`.
- Create an implicit global module fragment for the language linkage
  declarations inside a module purview.
    - If the language linkage lives inside the scope of an export decl,
      the created modules is marked as exported to outer modules.
- In fact, Sema will only create at most 2 implicit global module
  fragments to avoid creating a lot of unnecessary modules in the edging
case.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D144367

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index ccb983d..2aae41f 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -765,8 +765,10 @@ static StringRef ModuleKindName(Module::ModuleKind MK) {
     return "Partition Implementation";
   case Module::ModuleHeaderUnit:
     return "Header Unit";
-  case Module::GlobalModuleFragment:
+  case Module::ExplicitGlobalModuleFragment:
     return "Global Module Fragment";
+  case Module::ImplicitGlobalModuleFragment:
+    return "Implicit Module Fragment";
   case Module::PrivateModuleFragment:
     return "Private Module Fragment";
   }