Remove the unnecessary is_nothrow_swappable condition in swap.
Thanks to @lewissbaker who pointed out the unnecessary condition in
https://reviews.llvm.org/D81954#inline-756872. Since this codepath does not
make use of swap anyway (that codepath is a different branch), we can safely
remove this condition and produce better codegen when all types are nothrow
movable but are potentially-throwing swappable.
See codegen in https://gcc.godbolt.org/z/uDFZjz
Reviewed By: ldionne, #libc
Differential Revision: https://reviews.llvm.org/D83274