[cmake] Make include(GNUInstallDirs) always below project(..)

Its defaulting logic must go after `project(..)` to work correctly,  but `project(..)` is often in a standalone condition making this
awkward, since the rest of the condition code may also need GNUInstallDirs.

The good thing is there are the various standalone booleans, which I had missed before. This makes splitting the conditional blocks less awkward.

Reviewed By: arichardson, phosek, beanz, ldionne, #libunwind, #libc, #libc_abi

Differential Revision: https://reviews.llvm.org/D117639