Reinstate r316025, reverted in r316029.

Original commit message:
"[cmake] Use find_package to discover zlib

This allows us to use standard cmake utilities to point to non-system zlib
locations.

Patch by Oksana Shadura and me (D39002)."

The new patch brings back the old behavior in the cases where find_package
cannot find zlib.

llvm-svn: 316150