CMake: Enable WMO for Release builds

The new interpreter heavily benefits from WMO and it's even slower than
the old one without it. This change enables WMO for Release builds for
CMake builds as it's not enabled by default.

Use `CMAKE_Swift_COMPILATION_MODE` if CMake 3.29 or newer is used, and
fall back to `add_compile_options` if older.
https://cmake.org/cmake/help/latest/prop_tgt/Swift_COMPILATION_MODE.html

Author: kateinoigakukun

CMakeLists.txt

@@ -7,6 +7,13 @@ project(WasmKit LANGUAGES C Swift)
 set(SWIFT_VERSION 5)
 set(CMAKE_Swift_LANGUAGE_VERSION ${SWIFT_VERSION})
 
+# Enable whole module optimization for Release or RelWithDebInfo builds.
+if(POLICY CMP0157)
+  set(CMAKE_Swift_COMPILATION_MODE $<IF:$<CONFIG:Release,RelWithDebInfo>,wholemodule,incremental>)
+else()
+  add_compile_options($<$<AND:$<COMPILE_LANGUAGE:Swift>,$<CONFIG:Release,RelWithDebInfo>>:-wmo>)
+endif()
+
 if(CMAKE_VERSION VERSION_LESS 3.21)
   get_property(parent_dir DIRECTORY PROPERTY PARENT_DIRECTORY)
   if(NOT parent_dir)