[CMake] Ensure module tracing is off during configuration

Such module usages are not relevant for the final build, they are used
only to detect the capabilities of the compiler.

This generalizes swiftlang#68453, and would be needed for Apple internal
configurations that set `SWIFT_LOADED_MODULE_TRACE_FILE` when building
the compiler.

Addresses rdar://124954349

Author: edymtt

CMakeLists.txt

@@ -100,6 +100,11 @@ include(FetchContent)
 # optional until we have a bootstrap story.
 check_language(Swift)
 if(CMAKE_Swift_COMPILER)
+  # we are not interested in logging any Swift module used
+  # when configuring the build system -- those are not useful
+  # since they will not contribute to the build of the compiler itself
+  unset(ENV{SWIFT_LOADED_MODULE_TRACE_FILE})
+
   enable_language(Swift)
   set(DEFAULT_SWIFT_MIN_RUNTIME_VERSION "${CMAKE_Swift_COMPILER_VERSION}")
 else()

cmake/modules/SwiftImplicitImport.cmake

@@ -3,8 +3,6 @@ function(swift_supports_implicit_module module_name out_var)
   file(WRITE "${CMAKE_BINARY_DIR}/tmp/empty-check-${module_name}.swift" "")
   execute_process(
     COMMAND
-      ${CMAKE_COMMAND}
-      -E env --unset=SWIFT_LOADED_MODULE_TRACE_FILE
       "${CMAKE_Swift_COMPILER}"
       -Xfrontend -disable-implicit-${module_name}-module-import
       -Xfrontend -parse-stdlib