swift234

[cmake] Use already defined ${product}_PATH_TO_LLVM_{SOURCE,BUILD} values instead of re-setting it later ot the same value.

Before this patch, we would always set ${product}_PATH_TO_LLVM_{SOURCE,BUILD} to
a value taken from llvm-config. But, turns out we had generated llvm-config
using these values. Thus llvm-config is just giving us back values we already
have.

This commit changes these variables to be precondition variables and doesn't
cause them to be reset.

rdar://26154980