Several PSTL headers included by <algorithm> are missing includes for things they use.
Switch some quoted includes to angle includes.
(Issues found from running `check-cxx` with https://reviews.llvm.org/D144322)
Reviewed By: ldionne, #libc
Differential Revision: https://reviews.llvm.org/D150085
This removes the need for a custom libc++ build to have a basic set of PSTL algorithms.
Reviewed By: ldionne, #libc
Spies: miyuki, libcxx-commits, arichardson
Differential Revision: https://reviews.llvm.org/D149624