[pstl] Remove various warnings in the pstl headers and tests

- unused parameter warnings
- don't use single-letter template parameter names, like we do in libc++
- sign-comparison warnings
- unused variables in the tests
- unused local typedefs in the tests
- the use of #include_next
- field reordering in the tests
- unused lambda captures

Note that the rationale for why the static_casts to unsigned are OK is
that last - first must always be non-negative, since [first, last) is
a valid range.

llvm-svn: 362148