fix(lower): diagnose .* on a non-pointer instead of codegen panic

`lowerDerefExpr` left the deref's result type `.unresolved` when the
operand wasn't a pointer (e.g. a stale `value.*` after a parameter
changed from `*T` to `T`), and emitted the `.deref` anyway. That
unresolved type slipped through to emit_llvm's "unresolved type reached
LLVM emission" panic with no source location.

Now it emits a clean diagnostic at the deref site
("cannot dereference with `.*`: 'T' is not a pointer") and recovers.
Regression: examples/254-deref-non-pointer-reject.sx.

examples/254-deref-non-pointer-reject.sx

@@ -0,0 +1,14 @@
+// `.*` on a non-pointer must be a clean compile diagnostic, NOT a codegen
+// panic. Regression: a stale `value.*` (e.g. after a parameter changed from
+// `*T` to `T` by value) used to lower a `.deref` with an `.unresolved` result
+// type, which slipped through to emit_llvm's "unresolved type reached LLVM
+// emission" panic with no source location. `lowerDerefExpr` now diagnoses it.
+// Expected: a clean error pointing at the deref; exit 1.
+
+Point :: struct { x: s32; y: s32; }
+
+main :: () -> s32 {
+    p : Point = .{ x = 3, y = 4 };
+    q := p.*;          // ERROR: `p` is a Point value, not a pointer
+    return q.x;
+}