[flang] Fix a warning

This patch fixes:

  flang-rt/lib/runtime/descriptor.cpp:30:19: error: first argument in
  call to 'memcpy' is a pointer to non-trivially copyable type
  'Fortran::runtime::Descriptor' [-Werror,-Wnontrivial-memcall]

Author: kazutakahirata

flang-rt/lib/runtime/descriptor.cpp

@@ -27,7 +27,7 @@ RT_OFFLOAD_API_GROUP_BEGIN
 RT_API_ATTRS Descriptor::Descriptor(const Descriptor &that) { *this = that; }
 
 RT_API_ATTRS Descriptor &Descriptor::operator=(const Descriptor &that) {
-  runtime::memcpy(this, &that, that.SizeInBytes());
+  runtime::memcpy(reinterpret_cast<void *>(this), &that, that.SizeInBytes());
   return *this;
 }