Update CMakeLists.txt

Author: compnerd

SwiftCompilerSources/CMakeLists.txt

@@ -232,6 +232,9 @@ else()
 
   # TODO: generate this dynamically through the modulemap; this cannot use `sed`
   # as that is not available on all paltforms (e.g. Windows).
+  #
+  # step 1: generate a dummy source file, which just includes all headers
+  # defined in include/swift/module.modulemap
   file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/HeaderDependencies.cpp"
        "\
 #include \"Basic/BridgedSwiftObject.h\" \