[CMake] Add LIBXML2_DEFINITIONS when testing for symbol existance

[zero9178]

Summary

Currently when linking LLVM against Libxml2, a simple check is performed to check whether it can be linked successfully. This check currently adds the include directories and the libraries for libxml2, but not definitions found by the config.

This causes issues on Windows when trying to link against a static libxml2. Libxml2 requires LIBXML_STATIC to be defined in the preprocessor to be able to link statically. This definition is put into LIBXML2_DEFINITIONS in the cmake config, but not properly forwarded to check_symbol_exists leading to it failing as it could not find xmlReadMemory in a DLL.

This patch simply appends the content of LIBXML2_DEFINITIONS to the symbol check definitions, fixing the issue.

[phosek]

LGTM

llvm/cmake/config-ix.cmake

elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
     find_package(LibXml2)
   endif()
   if(LibXml2_FOUND)
     # Check if libxml2 we found is usable; for example, we may have found a 32-bit
     # library on a 64-bit system which would result in a link-time failure.
     cmake_push_check_state()
     list(APPEND CMAKE_REQUIRED_INCLUDES ${LIBXML2_INCLUDE_DIRS})
     list(APPEND CMAKE_REQUIRED_LIBRARIES ${LIBXML2_LIBRARIES})
+    list(APPEND CMAKE_REQUIRED_DEFINITIONS ${LIBXML2_DEFINITIONS})
     check_symbol_exists(xmlReadMemory libxml/xmlreader.h HAVE_LIBXML2)
     cmake_pop_check_state()
     if(LLVM_ENABLE_LIBXML2 STREQUAL FORCE_ON AND NOT HAVE_LIBXML2)
       message(FATAL_ERROR "Failed to configure libxml2")
     endif()
   endif()
   set(LLVM_ENABLE_LIBXML2 "${HAVE_LIBXML2}")
 endif()