[libc++] Use _Lazy in tuple constructors

This reduces the number of instantiations and also avoid blowing up
past the fold-expression limit of Clang.

This is NOT a general statement that we should strive to stay within
Clang's (sometimes way too small) limits, however in this case the
change will reduce the number of template instantiations while at the
same time doing that, which is good.

Differential Revision: https://reviews.llvm.org/D132509