[cfe] [test] Fix detecting LLVM zlib support in stand-alone builds

[mgorny]

Summary

Fix the test run to declare missing HAVE_LIBZ value in stand-alone builds, using the LLVM_ENABLE_ZLIB that is exported in LLVMConfig.cmake.

When using in-tree builds, HAVE_LIBZ is declared in cmake/config-ix.cmake as a result of LLVM's CMake checks. When building stand-alone, this value is not available and as a result caused clang to wrongly assume that LLVM was built without zlib support.

To fix it, set it to the value of LLVM_ENABLE_ZLIB. While this variable is originally used to control the user preference, LLVM updates its value to 0 if zlib checks fail. Therefore, we can use it to reliably determine whether LLVM was built with zlib support or not.

[mgorny]

For the record: I've tested the patch with and without zlib installed, using in-tree and stand-alone builds.

[mgorny]

Gentle ping.

[mgorny]

Non-gentle ping?

[beanz]

LGTM!

test/CMakeLists.txt

 # Test runner infrastructure for Clang. This configures the Clang test trees
 # for use by Lit, and delegates to LLVM's lit test handlers.
 
 if (CMAKE_CFG_INTDIR STREQUAL ".")
   set(LLVM_BUILD_MODE ".")
 else ()
   set(LLVM_BUILD_MODE "%(build_mode)s")
 endif ()
 
 string(REPLACE ${CMAKE_CFG_INTDIR} ${LLVM_BUILD_MODE} CLANG_TOOLS_DIR ${LLVM_RUNTIME_OUTPUT_INTDIR})
 
+if(CLANG_BUILT_STANDALONE)
+  # Set HAVE_LIBZ according to recorded LLVM_ENABLE_ZLIB value. This
+  # value is forced to 0 if zlib was not found, so it is fine to use it
+  # instead of HAVE_LIBZ (not recorded).
+  if(LLVM_ENABLE_ZLIB)
+    set(HAVE_LIBZ 1)
+  endif()
+endif()
+
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
   )
 
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg