config-ix.cmake: Search for CMAKE_XCRUN before using it.
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@294811 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm