Add __traits(vtblSymbol), __traits(interfaceSymbol)

[TurkeyMan]

This is an alternative take on #23940, it creates an artificial slice SymbolSlice!T instead of a real proper one, that way the compiler doesn't need any extra tricks.
The disadvantage is that expressions like typeof(__traits(vtblSymbol, C)) and friends will not report (void*)[], but instead SymbolSlice!(void*), which means they may not interact completely naturally with some set of meta, but these are super-niche things, and really just to facilitate lib-defined ClassInfo; so I think we can live with that imperfection.
__traits(vtblSymbol, C)[] would cause it to present as a natural slice and work through any meta correctly.

[thewilsonator]

try closing this and reopening this PR from your own fork. That should kill off those failing bootstrap CI runs.

[github-actions]

DMD perf check

Metric	Base	PR	Δ
compile hello.d (instr)	210.3 M	210.6 M	+0.146%
compile hello.d -O -release (instr)	227.9 M	228.2 M	+0.158%
compile Phobos (instr)	4,891.2 M	4,899.1 M	+0.160%
compile Phobos codegen (instr)	1,426.1 M	1,429.7 M	+0.253%
compile vibe.d (instr)	14,258.0 M	14,278.6 M	+0.144%

Breakdown — compile hello.d

Phase (wall, self time)	Base	PR	Δ
sema1	8.4 ms	9.5 ms	+13.36%
sema_other	12.5 ms	13.5 ms	+7.24%
parse	24.0 ms	24.6 ms	+2.54%
sema3	4.4 ms	4.2 ms	-5.41%
codegen	1.6 ms	1.8 ms	+9.84%

Breakdown — compile Phobos

+7.8 M instructions: frontend +4.2 M (+0.12%), codegen +3.6 M (+0.25%)

Phase (wall, self time)	Base	PR	Δ
codegen	340 ms	325 ms	-4.54%
sema3	412 ms	415 ms	+0.76%
sema1	164 ms	161 ms	-1.40%
parse	82.1 ms	84.1 ms	+2.50%
sema_other	176 ms	178 ms	+1.05%
ctfe	9.9 ms	9.4 ms	-4.48%
inline	4.2 ms	4.1 ms	-3.89%
sema2	0.8 ms	0.9 ms	+6.40%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	210.3 M	210.6 M	+0.146%
compile hello.d -O -release (instr)	227.9 M	228.2 M	+0.158%
compile Phobos (instr)	4,891.2 M	4,899.1 M	+0.160%
compile Phobos codegen (instr)	1,426.1 M	1,429.7 M	+0.253%
compile vibe.d (instr)	14,258.0 M	14,278.6 M	+0.144%
dmd binary size (stripped)	8.15 MB	8.15 MB	-0.09%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	44.10 MB	44.18 MB	+0.18%
peak RSS (compile Phobos)	605.1 MB	605.4 MB	+0.04%
peak RSS (compile vibe.d)	1858 MB	1857 MB	-0.04%
page faults (compile hello.d)	8,897	8,910	+0.15%
page faults (compile Phobos)	151,179	150,732	-0.30%
page faults (compile vibe.d)	464,216	466,100	+0.41%
compile dmd itself (wall)	10.0 s	10.1 s	+1.27%
compile hello.d (wall)	51.0 ms	53.6 ms	+5.03%
compile Phobos (wall)	1,189 ms	1,177 ms	-0.95%

_{0aab637 vs merge-base 92ac966 · about these metrics}