This is important if the user has multiple Xcodes installed on their
system -- we use xcodebuild to do a version check, and therefore we need
to make sure we match the actual build command.

Reported by Howard Ling!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182498 91177308-0d34-0410-b5e6-96231b3b80d8