Revert "[modules] Enable cxxmodules for LLVM and libstdc++."

This reverts commit 86c95d9.

We are not quite ready to do this. Our PR infrastructure gave
green light while it shouldn't have.

Author: vgvassilev

interpreter/CMakeLists.txt

@@ -93,7 +93,10 @@ if(cxxmodules)
   # because the cxxmodules builds in llvm have different build dependency order.
   string(REPLACE "${ROOT_CXXMODULES_CXXFLAGS}" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
   string(REPLACE "${ROOT_CXXMODULES_CFLAGS}" "" CMAKE_C_FLAGS ${CMAKE_C_FLAGS})
-  set (LLVM_ENABLE_MODULES ON CACHE BOOL "Override the default LLVM_ENABLE_MODULES option value." )
+  if(libcxx)
+    # FIXME: We cannot build LLVM/clang with modules on with libstdc++, yet.
+    set (LLVM_ENABLE_MODULES ON CACHE BOOL "Override the default LLVM_ENABLE_MODULES option value." )
+  endif(libcxx)
 endif(cxxmodules)
 
 if(gcctoolchain)