[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, 1 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 9b2982e..2ffb0db 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -397,6 +397,7 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   if (Interface) {
 
     VisibleModules.setVisible(Interface, ModuleLoc);
+    VisibleModules.makeTransitiveImportsVisible(Interface, ModuleLoc);
 
     // Make the import decl for the interface in the impl module.
     ImportDecl *Import = ImportDecl::Create(Context, CurContext, ModuleLoc,