cmake: correctly set NDEBUG for imported headers in SwiftCompilerSources

So that it's consistent with C++ sources

rdar://110363377
(cherry picked from commit fa2ac84)

Author: eeckstein

SwiftCompilerSources/CMakeLists.txt

@@ -87,6 +87,10 @@ function(add_swift_compiler_modules_library name)
     list(APPEND swift_compile_options "-O" "-cross-module-optimization")
   endif()
 
+  if(NOT LLVM_ENABLE_ASSERTIONS)
+    list(APPEND swift_compile_options "-Xcc" "-DNDEBUG")
+  endif()
+
   if(NOT SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT)
     list(APPEND swift_compile_options "-Xfrontend" "-disable-legacy-type-info")
   endif()