[CMake] Prevent rebuilds of pure swift libraries and executables

`.swiftmodule` is listed as an output of pure swift libraries, but it
isn't updated if there's been no changes (just like the library and
its object files). Add it to the touch hack as well.

Swift *executables* also list `.swiftmodule` as an output, even though
one isn't generated at all. Add a new POST_BUILD command to touch it
regardless.

Author: bnbarham

cmake/modules/AddPureSwift.cmake

@@ -163,7 +163,7 @@ function(add_pure_swift_host_library name)
   add_custom_command(
       TARGET ${name}
       POST_BUILD
-      COMMAND "${CMAKE_COMMAND}" -E touch_nocreate $<TARGET_FILE:${name}> $<TARGET_OBJECTS:${name}>
+      COMMAND "${CMAKE_COMMAND}" -E touch_nocreate $<TARGET_FILE:${name}> $<TARGET_OBJECTS:${name}> "${SWIFT_HOST_LIBRARIES_DEST_DIR}/${name}.swiftmodule" "${CMAKE_CURRENT_BINARY_DIR}/${name}.swiftmodule"
       COMMAND_EXPAND_LISTS
       COMMENT "Update mtime of library outputs workaround")
 
@@ -304,6 +304,24 @@ function(add_pure_swift_host_tool name)
   target_include_directories(${name} PUBLIC
     ${SWIFT_HOST_LIBRARIES_DEST_DIR})
 
+  # Workaround to touch the library and its objects so that we don't
+  # continually rebuild (again, see corresponding change in swift-syntax).
+  add_custom_command(
+      TARGET ${name}
+      POST_BUILD
+      COMMAND "${CMAKE_COMMAND}" -E touch_nocreate $<TARGET_FILE:${name}> $<TARGET_OBJECTS:${name}>
+      COMMAND_EXPAND_LISTS
+      COMMENT "Update mtime of executable outputs workaround")
+
+  # Even worse hack - ${name}.swiftmodule is added as an output, even though
+  # this is an executable target. Just touch it all the time to avoid having
+  # to rebuild it every time.
+  add_custom_command(
+      TARGET ${name}
+      POST_BUILD
+      COMMAND "${CMAKE_COMMAND}" -E touch "${CMAKE_CURRENT_BINARY_DIR}/${name}.swiftmodule"
+      COMMAND_EXPAND_LISTS
+      COMMENT "Update mtime of executable outputs workaround")
   # Export this target.
   set_property(GLOBAL APPEND PROPERTY SWIFT_EXPORTS ${name})
 endfunction()