Revert "build: make LLVM_ENABLE_ZLIB a tri-bool for users"

This reverts commit e6c7ed6d2164a0659fd9f6ee44f1375d301e3cad.

This commit was an attempt to fix the build bots, but it still left the
clang-x64-windows-msvc bot in a broken state.

CMakeLists.txt

@@ -347,7 +347,7 @@ option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON
 
 option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
-set(LLVM_ENABLE_ZLIB "ON" CACHE STRING "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON")
+option(LLVM_ENABLE_ZLIB "Use zlib for compression/decompression if available." ON)
 
 set(LLVM_Z3_INSTALL_DIR "" CACHE STRING "Install directory of the Z3 solver.")
 

cmake/config-ix.cmake

@@ -117,6 +117,10 @@ endif()
 # Don't look for these libraries if we're using MSan, since uninstrumented third
 # party code may call MSan interceptors like strlen, leading to false positives.
 if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
+  if(LLVM_ENABLE_ZLIB)
+    find_package(ZLIB REQUIRED)
+  endif()
+
   # Don't look for these libraries on Windows.
   if (NOT PURE_WINDOWS)
     # Skip libedit if using ASan as it contains memory leaks.
@@ -502,21 +506,7 @@ else( LLVM_ENABLE_THREADS )
 endif()
 
 if(LLVM_ENABLE_ZLIB)
-  if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON)
   find_package(ZLIB REQUIRED)
-  else()
-    find_package(ZLIB)
-  endif()
-
-  if(ZLIB_FOUND)
-    set(LLVM_ENABLE_ZLIB "YES" CACHE STRING
-      "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON"
-      FORCE)
-  else()
-    set(LLVM_ENABLE_ZLIB "NO" CACHE STRING
-      "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON"
-      FORCE)
-  endif()
 endif()
 
 if (LLVM_ENABLE_DOXYGEN)

lib/Support/CMakeLists.txt

@@ -1,7 +1,4 @@
-if(LLVM_ENABLE_ZLIB)
 set(system_libs ${ZLIB_LIBRARY})
-endif()
-
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.