RunCMake/CXXModules: add a "deep-chain" test

This tests that transitive usages are propogated properly across long
import chains between targets.

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -131,6 +131,7 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(library library-static -DBUILD_SHARED_LIBS=OFF)
   run_cxx_module_test(generated)
   run_cxx_module_test(public-req-private)
+  run_cxx_module_test(deep-chain)
 endif ()
 
 # Tests which use named modules in shared libraries.

Tests/RunCMake/CXXModules/examples/deep-chain-stderr.txt

@@ -0,0 +1,9 @@
+CMake Warning \(dev\) at CMakeLists.txt:7 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/CXXModules/examples/deep-chain/CMakeLists.txt

@@ -0,0 +1,66 @@
+cmake_minimum_required(VERSION 3.24)
+project(cxx_modules_deep_chain CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+add_library(a STATIC)
+target_sources(a
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        a.cxx)
+target_compile_features(a PUBLIC cxx_std_20)
+
+add_library(b STATIC)
+target_sources(b
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        b.cxx)
+target_compile_features(b PUBLIC cxx_std_20)
+target_link_libraries(b PUBLIC a)
+
+add_library(c STATIC)
+target_sources(c
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        c.cxx)
+target_compile_features(c PUBLIC cxx_std_20)
+target_link_libraries(c PUBLIC b)
+
+add_library(d STATIC)
+target_sources(d
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        d.cxx)
+target_compile_features(d PUBLIC cxx_std_20)
+target_link_libraries(d PUBLIC c)
+
+add_library(e STATIC)
+target_sources(e
+  PUBLIC
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        e.cxx)
+target_compile_features(e PUBLIC cxx_std_20)
+target_link_libraries(e PUBLIC d)
+
+add_executable(exe)
+target_link_libraries(exe PRIVATE e)
+target_sources(exe
+  PRIVATE
+    main.cxx)
+
+add_test(NAME exe COMMAND exe)

Tests/RunCMake/CXXModules/examples/deep-chain/a.cxx

@@ -0,0 +1,6 @@
+export module a;
+
+export int a()
+{
+  return 0;
+}

Tests/RunCMake/CXXModules/examples/deep-chain/b.cxx

@@ -0,0 +1,7 @@
+export module b;
+import a;
+
+export int b()
+{
+  return a();
+}

Tests/RunCMake/CXXModules/examples/deep-chain/c.cxx

@@ -0,0 +1,7 @@
+export module c;
+import b;
+
+export int c()
+{
+  return b();
+}

Tests/RunCMake/CXXModules/examples/deep-chain/d.cxx

@@ -0,0 +1,7 @@
+export module d;
+import c;
+
+export int d()
+{
+  return c();
+}

Tests/RunCMake/CXXModules/examples/deep-chain/e.cxx

@@ -0,0 +1,7 @@
+export module e;
+import d;
+
+export int e()
+{
+  return d();
+}

Tests/RunCMake/CXXModules/examples/deep-chain/main.cxx

@@ -0,0 +1,6 @@
+import e;
+
+int main(int argc, char* argv[])
+{
+  return e();
+}