[C++20] [Modules] Correct the visibility of decls in implicit global module of other units in the same TU

See the test for the case. It is similar with
llvm@baa5b76

Author: ChuanqiXu9

clang/lib/Sema/SemaLookup.cpp

@@ -1614,7 +1614,7 @@ bool Sema::isUsableModule(const Module *M) {
 
   // Otherwise, the global module fragment from other translation unit is not
   // directly usable.
-  if (M->isGlobalModule())
+  if (M->isExplicitGlobalModule())
     return false;
 
   Module *Current = getCurrentModule();
@@ -1628,6 +1628,8 @@ bool Sema::isUsableModule(const Module *M) {
   // module should be visible to the decls in the implicit global module.
   if (Current->isImplicitGlobalModule())
     Current = Current->getTopLevelModule();
+  if (M->isImplicitGlobalModule())
+    M = M->getTopLevelModule();
 
   // If M is the module we're parsing or M and the current module unit lives in
   // the same module, M should be usable.

clang/test/Modules/visibility-for-implicit-global-module.cppm

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.interface.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/a.impl.cc -fmodule-file=a:interface=%t/a.pcm \
+// RUN:     -verify -fsyntax-only
+
+//--- a.interface.cppm
+export module a:interface;
+extern "C++" constexpr int a = 43;
+
+//--- a.impl.cc
+// expected-no-diagnostics
+module a:impl;
+import :interface;
+static_assert(a == 43);
+