re-land [C++20][Modules] Introduce an implementation module.

We need to be able to distinguish individual TUs from the same module in cases
where TU-local entities either need to be hidden (or, for some cases of ADL in
template instantiation, need to be detected as exposures).

This creates a module type for the implementation which implicitly imports its
primary module interface per C++20:
[module.unit/8] 'A module-declaration that contains neither an export-keyword
nor a module-partition implicitly imports the primary module interface unit of
the module as if by a module-import-declaration.

Implementation modules are never serialized (-emit-module-interface for an
implementation unit is diagnosed and rejected).

Differential Revision: https://reviews.llvm.org/D126959

1 files changed, 2 insertions, 0 deletions

diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index 2aae41f..05d9fc82 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -759,6 +759,8 @@ static StringRef ModuleKindName(Module::ModuleKind MK) {
     return "Module Map Module";
   case Module::ModuleInterfaceUnit:
     return "Interface Unit";
+  case Module::ModuleImplementationUnit:
+    return "Implementation Unit";
   case Module::ModulePartitionInterface:
     return "Partition Interface";
   case Module::ModulePartitionImplementation: