[C++20][Modules] Add testcases from section 10.2 dependent on header units.

[iains]

Summary

This adds in testcases reflecting the remaining example in section 10.2
of the C++20 standard.

[iains]

there might be more cases to add here, from later sections in the standard - but those might also have additional deps.

clang/test/Modules/cxx20-10-2-ex2.cpp

+// Based on C++20 10.2 example 2.
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -I %t \
+// RUN: -xc++-user-header std-10-2-ex2-c.h -o %t/std-10-2-ex2-c.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/std-10-2-ex2-tu1.cpp \
+// RUN:  -o  %t/X.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/std-10-2-ex2-tu2.cpp \
+// RUN: -fmodule-file=%t/std-10-2-ex2-c.pcm -fmodule-file=%t/X.pcm \
+// RUN: -pedantic-errors -verify -o  %t/M.pcm
+
+//--- std-10-2-ex2-b.h
+int f();
+
+//--- std-10-2-ex2-c.h
+int g();
+
+//--- std-10-2-ex2-tu1.cpp
+export module X;
+export int h();
+
+//--- std-10-2-ex2-tu2.cpp
+module;
+#include "std-10-2-ex2-b.h"
+
+export module M;
+import "std-10-2-ex2-c.h";
+import X;
+export using ::f, ::g, ::h; // OK
+struct S;                   // expected-note {{target of using declaration}}
+export using ::S;           // expected-error {{using declaration referring to 'S' with module linkage cannot be exported}}
+
+namespace N {
+export int h();
+static int h(int); // expected-note {{target of using declaration}}
+} // namespace N
+export using N::h; // expected-error {{using declaration referring to 'h' with internal linkage cannot be exported}}