[CMake] Simplify CMake handling for zlib

Rather than handling zlib handling manually, use find_package from CMake
to find zlib properly. Use this to normalize the LLVM_ENABLE_ZLIB,
HAVE_ZLIB, HAVE_ZLIB_H. Furthermore, require zlib if LLVM_ENABLE_ZLIB is
set to YES, which requires the distributor to explicitly select whether
zlib is enabled or not. This simplifies the CMake handling and usage in
the rest of the tooling.

This is a reland of abb00753 with all followup changes and fixes that
should address issues that were reported in PR44780.

Differential Revision: https://reviews.llvm.org/D79219