More P0202 constexpr work. This commit adds fill/fill_n/generate/generate_n/unique/unique_copy. I removed a specialization of fill_n that recognized when we were dealing with raw pointers and 1 byte trivially-assignable types and did a memset, because the compiler will do that optimization for us.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323050 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed