[CMAKE] Honor CMAKE_OSX_SYSROOT to compute include dir for libxml2

On MacOS, if CMAKE_OSX_SYSROOT is used and the user has command line tools
installed, we currently get the include path for libxml2 as
/usr/include/libxml2, instead of ${CMAKE_OSX_SYSROOT}/usr/include/libxml2.

Make it consistent on MacOS by prefixing ${CMAKE_OSX_SYSROOT} when
possible.

rdar://problem/41103601

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334746 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -148,7 +148,11 @@ if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
       find_package(LibXml2)
       if (LIBXML2_FOUND)
         set(LLVM_LIBXML2_ENABLED 1)
-        include_directories(${LIBXML2_INCLUDE_DIR})
+        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()
         set(LIBXML2_LIBS "xml2")
       endif()
     endif()