Make generated identifiers stable across compilation

[dkorpel]

There's a long standing problem where dmd uses global counters for generated identifiers, which depend on parsing / semantic analysis order and hence are not stable with separate compilation. This has been hacked around a bit by incorporating source locations in the identifier, but even that is not unique due to code generation constructs like static foreach and mixin(), so a fallback on a global counter was added with a partial fix that checks for the variable's scope. This PR removes the kludgy hashtable of counters system:

static struct Key { string locKey; string prefix; const(void)* parent; }
__gshared uint[Key] counters;

With stable indices based on adding an index to a Scope for static foreach (). There is now an O(n) fallback for overlapping mixin locations like:

mixin(`unittest { }`); mixin(`unittest { }`);

But that should be rare enough to be a performance problem.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#23207"