[libc++] Use __invoke throughout the PSTL algorithms

Right now we call user-provided functions like `__comp(a, b)` instead of `__invoke(__comp, a, b)`, like we do for serial algorithms. This is going to become a problem with range versions of PSTL algorithms if we want to share the same underlying implementation.

Start by locating the PSTL algorithms and compare their user-function calls with the serial algorithms that already use __invoke. Trace all affected PSTL call sites and verify that the algorithms consistently use __invoke without changing their behavior.

Written by the indexing model from the issue text.