[clang][cli] Round-trip cc1 arguments in assert builds

This patch enables cc1 argument round-trip for assert builds. It can be disabled by building clang with `-DCLANG_ROUND_TRIP_CC1_ARGS=OFF`.

This will be committed only if we reach consensus in https://lists.llvm.org/pipermail/cfe-dev/2021-February/067714.html.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D97462

clang/CMakeLists.txt

@@ -467,7 +467,8 @@ option(CLANG_ENABLE_STATIC_ANALYZER
 
 option(CLANG_ENABLE_PROTO_FUZZER "Build Clang protobuf fuzzer." OFF)
 
-option(CLANG_ROUND_TRIP_CC1_ARGS "Round-trip command line arguments in -cc1." OFF)
+option(CLANG_ROUND_TRIP_CC1_ARGS
+  "Round-trip command line arguments in -cc1." ${LLVM_ENABLE_ASSERTIONS})
 
 if(NOT CLANG_ENABLE_STATIC_ANALYZER AND CLANG_ENABLE_ARCMT)
   message(FATAL_ERROR "Cannot disable static analyzer while enabling ARCMT or Z3")