[lldb] Merge PlatformXXX::ResolveExecutable

The near-identical implementations of this function for posix-y
platforms were merged in r293910. PlatformWindows was left out of this
merge because at the time we did not have a suitable base class to sink
the code into. That is no longer true, so this commit finishes the job
by moving the code into RemoteAwarePlatform::ResolveExecutable.