Silence warning coming from LLVM due to memcpy into std::pair

interpreter/llvm/src/include/llvm/ADT/SmallVector.h:296:13:
  warning: ‘void* memcpy(void*, const void*, size_t)’ writing to an
  object of type ‘struct std::pair<clang::CXXMethodDecl*,
  const clang::FunctionProtoType*>’ with no trivial copy-assignment
  use copy-assignment or copy-initialization instead [-Wclass-memaccess]
       memcpy(Dest, I, (E - I) * sizeof(T));
       ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

core/dictgen/CMakeLists.txt

@@ -8,6 +8,8 @@
 # These files are linked into rootcling_stage1 and libCling. rootcling accesses
 # them through rootcling_impl.
 
+ROOT_ADD_COMPILE_OPTIONS(-Wno-class-memaccess)
+
 ROOT_GLOB_SOURCES(sources ${CMAKE_CURRENT_SOURCE_DIR}/src/*.cxx)
 
 ROOT_OBJECT_LIBRARY(Dictgen