reduce Green Dragon macOS build session filename length
reduce Green Dragon macOS build session filename length

The Green Dragon builder for macOS started failing yesterday with
session filenames that were too long.  This change modifies the
Xcode target that runs the test suite and specifies a shorter
session filename format.

rdar://27539818

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@276760 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb