Add a lock to PlatformPOSIX::DoLoadImage
Add a lock to PlatformPOSIX::DoLoadImage

Multiple threads could be calling into DoLoadImage concurrently,
only one should be allowed to create the UtilityFunction.

This is discussed upstream in https://reviews.llvm.org/D46733, but
this seems fine for now.

rdar://problem/40140766

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb