[lldb] Remove the global platform list

This patch moves the platform creation and selection logic into the
per-debugger platform lists. I've tried to keep functional changes to a
minimum -- the main (only) observable difference in this change is that
APIs, which select a platform by name (e.g.,
Debugger::SetCurrentPlatform) will not automatically pick up a platform
associated with another debugger (or no debugger at all).

I've also added several tests for this functionality -- one of the
pleasant consequences of the debugger isolation is that it is now
possible to test the platform selection and creation logic.

This is a product of the discussion at
<https://discourse.llvm.org/t/multiple-platforms-with-the-same-name/59594>.

Differential Revision: https://reviews.llvm.org/D120810