[CMake] zlib FORCE_ON overrides MSan

[phosek]

Summary

This restores the behavior removed in 1adc494 where setting
LLVM_ENABLE_ZLIB to FORCE_ON overrides MSan.

[hans]

lgtm

I don't know if anyone cares about the msan+zlib case, I just think it's important that FORCE_ON either enables zlib or raises an error. Looks like this patch ensures that.

[phosek]

This was combined into D79219.

llvm/cmake/config-ix.cmake

if(HAVE_LIBPTHREAD)
   # have it reported as '-l<lib>' instead of '-pthread'.
   # TODO: switch to -pthread once the rest of the build system can deal with it.
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
   set(THREADS_HAVE_PTHREAD_ARG Off)
   find_package(Threads REQUIRED)
   set(LLVM_PTHREAD_LIB ${CMAKE_THREAD_LIBS_INIT})
 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)
-    if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON)
-      find_package(ZLIB REQUIRED)
-    else()
-      find_package(ZLIB)
-    endif()
-    set(LLVM_ENABLE_ZLIB "${ZLIB_FOUND}")
-  endif()
+  elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
+  else()
+    set(ZLIB_FOUND FALSE)
 
+# 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.*")
   # Don't look for these libraries on Windows.
   if (NOT PURE_WINDOWS)
     # Skip libedit if using ASan as it contains memory leaks.
     if (LLVM_ENABLE_LIBEDIT AND HAVE_HISTEDIT_H AND NOT LLVM_USE_SANITIZER MATCHES ".*Address.*")
       check_library_exists(edit el_init "" HAVE_LIBEDIT)
     else()
       set(HAVE_LIBEDIT 0)
     endif()
if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
         if ((CMAKE_OSX_SYSROOT) AND (EXISTS ${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR}))
           include_directories(${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR})
         else()
           include_directories(${LIBXML2_INCLUDE_DIR})
         endif()
       endif()
     endif()
   endif()
-else()
-  set(LLVM_ENABLE_ZLIB FALSE)
 endif()
 
 if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
   message(FATAL_ERROR "Failed to congifure libxml2")
 endif()
 
 check_library_exists(xar xar_open "" HAVE_LIBXAR)
 if(HAVE_LIBXAR)