Reorganise the thread locals for the backtracing code in the `Runtime` module so
that there’s only one set of them, with everything else hanging off that. This
also reduces code duplication and consumption of thread local variable space.
Fix the `DefaultSymbolLocator`’s shared instance to be thread local.
rdar://171432566
This is better than the `alternativePaths` mechanism because it means that
a client of the `Runtime` module can do its own thing, which might even
mean fetching things from a remote server or decompressing files or any
other thing we might find useful.
rdar://170642627
