Fix #23239: give the nested tempinst a chance of instantiation

[gulugulubing]

Try to fix #23239:

The instance relationship was not immediately clear to me, so I used an LLM to help draw this graph:

module
│
├── template Tmpl                  ← original declaration, parent = module
│   └── template touch             ← original declaration, parent = Tmpl
│
├── Tmpl!()_primary                ← instance (1st, created inside static assert)
│   │   parent = module, inst = self, minst = root (after swap)
│   │
│   ├── int data                   ← copy declaration, parent = Tmpl!()
│   └── template touch             ← copy declaration, parent = Tmpl!()  ← key!
│
├── touch!int                      ← instance (1st, created inside static assert)
│   │   parent = module, inst = self, minst = null  ← BUG！
│   │   tempdecl ──────────────→ template touch (the copy inside Tmpl!())
│   │   tinst = null             (cleared by StaticAssert)
│   │
│   ├── static this()              ← module ctor, parent = touch!int
│   └── enum touch = true          ← eponymous member
│
├── Tmpl!()_secondary              ← instance (2nd, created in main())
│       inst → Tmpl!()_primary, minst = null (after swap)
│       (no members — secondary is just a record, not a copy)
│
└── main()                         ← function declaration

The root cause: touch!int is only instantiated once, inside static assert. StaticAssert::semantic2() sets sc.minst = null before evaluating the condition, so touch!int gets minst = null (speculative). Later when Tmpl!() is used at runtime and gets swapped to a root instance, the nested touch!int is left behind with minst = null, and needsCodegen() skips it entirely — including its static this().

The fix: after the swap makes Tmpl!() a root instance, scan the root module's members for any speculative TemplateInstances whose enclosing chain resolves to the newly-rooted instance. There is no direct backlink from TemplateDeclaration to TemplateInstance, so we filter candidates by checking nested.tempdecl.isInstantiated(). Once found, set nested.minst = root.

[gulugulubing]

It looks break the phobo, marked as draft first.

[gulugulubing]

It looks good now, by moving the fix logic to needsCodegen(). The previous approach modified minst during semantic analysis, which could interfere with module-level template instantiation order and break Phobos builds.