libcxxabi: Suppress LLVM_ENABLE_MODULES

[chapuni]

Summary

It's copypasto from libcxx.
There's no serious issue. I'd like just to keep module.cache clean for in-tree libcxx builds.

[jroelofs]

Does the unwinder need this too?

[chapuni]

@jroelofs Please do. libunwind emits unique module caches.

module.cache/1JROMJZVPMD3F/std-WU7UZNPX10U5.pcm
module.cache/1JROMJZVPMD3F/std_config-WU7UZNPX10U5.pcm
module.cache/1JROMJZVPMD3F/_Builtin_stddef_max_align_t-1P37D07AX0QU6.pcm

They are not used outside the tree.

libcxxabi/trunk/CMakeLists.txt

if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)
   endif()
   if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
     message(FATAL_ERROR "The options LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY"
                         " and LIBCXXABI_HAS_EXTERNAL_THREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()
 endif()
 
+if (LLVM_ENABLE_MODULES)
+  # Ignore that the rest of the modules flags are now unused.
+  add_compile_flags_if_supported(-Wno-unused-command-line-argument)
+  add_compile_flags(-fno-modules)
+endif()
+
 set(LIBCXXABI_HAS_UNDEFINED_SYMBOLS OFF)
 if ((NOT LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS)
     OR (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY AND LIBCXXABI_ENABLE_SHARED)
     OR MINGW)
   set(LIBCXXABI_HAS_UNDEFINED_SYMBOLS ON)
 endif()
 
 if (LIBCXXABI_HAS_UNDEFINED_SYMBOLS)