Sema: introduce a type resolution queue

That happens after a function body is analyzed. This prevents circular
dependency compile errors and yet a way to mark types that need to be
fully resolved before a given function is sent to the codegen backend.

src/Module.zig

@@ -4837,6 +4837,9 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
 
     // Finally we must resolve the return type and parameter types so that backends
     // have full access to type information.
+    // Crucially, this happens *after* we set the function state to success above,
+    // so that dependencies on the function body will now be satisfied rather than
+    // result in circular dependency errors.
     const src: LazySrcLoc = .{ .node_offset = 0 };
     sema.resolveFnTypes(&inner_block, src, fn_ty_info) catch |err| switch (err) {
         error.NeededSourceLocation => unreachable,
@@ -4847,6 +4850,20 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
         else => |e| return e,
     };
 
+    // Similarly, resolve any queued up types that were requested to be resolved for
+    // the backends.
+    for (sema.types_to_resolve.items) |inst_ref| {
+        const ty = sema.getTmpAir().getRefType(inst_ref);
+        sema.resolveTypeFully(&inner_block, src, ty) catch |err| switch (err) {
+            error.NeededSourceLocation => unreachable,
+            error.GenericPoison => unreachable,
+            error.ComptimeReturn => unreachable,
+            error.ComptimeBreak => unreachable,
+            error.AnalysisFail => {},
+            else => |e| return e,
+        };
+    }
+
     return Air{
         .instructions = sema.air_instructions.toOwnedSlice(),
         .extra = sema.air_extra.toOwnedSlice(gpa),