[C++20][Modules] Complete implementation of module.import p7.

The following test fails to compile TU b.cpp because we are not making the transitively imported modules visible (per [module.import]/p7)

```
a.cppm:
export module a;

export int foo() {
   return 42;
}

b.cppm:
export module b;
import a;

export int bar();

b.cpp:
module b;

int bar() {
   return foo();
}

clang++ -c -std=c++2b -fmodule-output a.cppm
clang++ -c -std=c++2b -fmodule-output -fprebuilt-module-path=. b.cppm
clang++ -c -std=c++2b -fprebuilt-module-path=. b.cpp
b.cpp:4:12: error: declaration of 'foo' must be imported from module 'a' before it is required
   return foo();
```

This is fixed by the following patch (which also addresses a FIXME in basic.def.odr/p6.cppm).

Differential Revision: https://reviews.llvm.org/D152746

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
index 3f9b8d0..2bdbe8d2 100644
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -695,6 +695,14 @@ void VisibleModuleSet::setVisible(Module *M, SourceLocation Loc,
   VisitModule({M, nullptr});
 }
 
+void VisibleModuleSet::makeTransitiveImportsVisible(Module *M,
+                                                    SourceLocation Loc,
+                                                    VisibleCallback Vis,
+                                                    ConflictCallback Cb) {
+  for (auto *I : M->Imports)
+    setVisible(I, Loc, Vis, Cb);
+}
+
 ASTSourceDescriptor::ASTSourceDescriptor(Module &M)
     : Signature(M.Signature), ClangModule(&M) {
   if (M.Directory)