Tests/CXXModules: add a test which scans a PCH-using source

This tests that PCH usage works with scanning logic.

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -165,6 +165,8 @@ if (RunCMake_GENERATOR MATCHES "Ninja")
   run_cxx_module_test_target(ninja-cmp0154 "${ninja_cmp0154_target}")
 endif ()
 
+run_cxx_module_test(scan-with-pch)
+
 # Tests which use named modules.
 if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(simple)

Tests/RunCMake/CXXModules/examples/scan-with-pch/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.28)
+project(cxx_modules_scan_with_pch CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+add_executable(simple)
+target_sources(simple
+  PRIVATE
+    main.cxx)
+target_compile_features(simple PUBLIC cxx_std_20)
+target_precompile_headers(simple PRIVATE pch.h)
+
+add_test(NAME simple COMMAND simple)

Tests/RunCMake/CXXModules/examples/scan-with-pch/main.cxx

@@ -0,0 +1,8 @@
+#ifndef from_pch
+#  error "pch not present"
+#endif
+
+int main(int argc, char* argv[])
+{
+  return 0;
+}

Tests/RunCMake/CXXModules/examples/scan-with-pch/pch.h

@@ -0,0 +1 @@
+#define from_pch