[pstl] Avoid shadowing explicit lambda capture with lambda parameter

Summary:
Recent Clangs give an error for this. Note that the size of this diff is
caused by running clang-format on the result of removing the captures.
I guess we'll see how well that works for us.

Reviewers: jfb, rodgert

Subscribers: jkorous, dexonsmith, libcxx-commits, mclow.lists

Differential Revision: https://reviews.llvm.org/D55945

llvm-svn: 350544