[libc++] Use this in lamba capture in <latch>

"&" seemed to be used in a situation where perhaps it's not the best option.
Other libc++ modules make use of [this] when calling functions from the same class.
[this] would be the appropriate lambda capture specifier to use in this situation.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D154358