[lldb/Platform] Prepare decouple instance and plugin names

This patch changes the return value of Platform::GetName() to a
StringRef, and uses the opportunity (compile errors) to change some
callsites to use GetPluginName() instead. The two methods still remain
hardwired to return the same thing, but this will change once the ideas
in
<https://discourse.llvm.org/t/multiple-platforms-with-the-same-name/59594>
are implemented.

Differential Revision: https://reviews.llvm.org/D119146