build: only pass -UNDEBUG when compiling C/C++

This patch limits adding -UNDEBUG to C and C++ files so that projects
can include files compiled with compilers that don't recognize this
argument (Swift e.g.).

add_definitions does not expand generators, hence the change from
add_definitions to add_compile_options.

Patch by Evan Wilde!

Author: compnerd

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -57,7 +57,10 @@ if( LLVM_ENABLE_ASSERTIONS )
   # On non-Debug builds cmake automatically defines NDEBUG, so we
   # explicitly undefine it:
   if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
-    add_definitions( -UNDEBUG )
+    # NOTE: use `add_compile_options` rather than `add_definitions` since
+    # `add_definitions` does not support generator expressions.
+    add_compile_options($<$<COMPILE_LANGUAGE:C,CXX>:-UNDEBUG>)
+
     # Also remove /D NDEBUG to avoid MSVC warnings about conflicting defines.
     foreach (flags_var_to_scrub
         CMAKE_CXX_FLAGS_RELEASE