[clang] Pass option directly to command. NFC

This code was written back when LLVM's minimum required CMake version
was 2.8.8, and I assume ExternalProject_Add_Step didn't take this option
at that point. It does now though, so we should just use the option.
Setting the _EP_* property is entirely equivalent (and is in fact how
these commands behave internally), but that also feels like an internal
implementation detail we shouldn't be relying on.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D99322