config-ix.cmake: Search for CMAKE_XCRUN before using it.

This was previously searched in CMakeLists.txt unconditionally but as of
r294371 it is only searched in some circumstances. Repeating the search
in config-ix.cmake to make this robust and hopefully fix the macOS
Asan+Ubsan jenkins build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294811 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed