Avoid CMake's default -O3 for Release builds

Author: kubamracek

CMakeLists.txt

@@ -629,6 +629,8 @@ if("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQU
   set(SWIFT_COMPILER_IS_MSVC_LIKE TRUE)
 endif()
 
+# Avoid CMake's default -O3 for Release builds.
+set(CMAKE_CXX_FLAGS_RELEASE "-O2 -DNDEBUG")
 if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
   _compute_lto_flag("${SWIFT_TOOLS_ENABLE_LTO}" _lto_flag_out)
   if(_lto_flag_out)