[ADT] Just give up on GCC, I can't fix this.

While the memmove workaround fixed it for GCC 6.3. GCC 4.8 and GCC 7.1
are still broken. I have no clue what's going on, just blacklist GCC for
now.

Needless to say this code is ubsan, asan and msan-clean.

llvm-svn: 322862