lang: fix struct-field null/undef over-store (issue 0154)

Assigning null/--- to a struct field picked up a leaked enclosing
target_type (the function's return type, set for the whole body), so
constNull/constUndef built a whole-struct-typed value. The oversized
store overran the field's slot and clobbered the saved frame pointer,
so the function returned to 0x0. Surfaced building a by-value-returned
struct whose array field precedes a pointer field (Scheduler.init()).

Fix: add null_literal/undef_literal to the needs_target switch in
lowerAssignment so the field's own type is used. Regression:
examples/types/0193-types-sret-array-before-pointer.sx.

examples/types/0193-types-sret-array-before-pointer.sx

@@ -0,0 +1,28 @@
+// Assigning `null` / `---` to a struct field must store a FIELD-typed value,
+// not pick up an enclosing `target_type` (e.g. the function's return type while
+// lowering its body) — which would build a whole-struct-typed null and emit an
+// oversized store that overruns the field's slot.
+//
+// Regression (issue 0154): `s.p = null` inside a struct-returning fn used to
+// store a 32-byte struct `zeroinitializer` through the pointer field's GEP,
+// clobbering the saved x29/x30 so the fn `ret`'d to 0x0. The array-before-
+// pointer order puts the pointer at a non-zero offset, pushing the over-store
+// off the end of the alloca. Fixed by adding null/undef literals to
+// `needs_target` in lowerAssignment.
+#import "modules/std.sx";
+S :: struct {
+    arr: [2]u64;   // fixed-array field FIRST
+    p:   *i64;     // pointer field AFTER the array
+    n:   i64;
+}
+mk :: () -> S {
+    s : S = ---;
+    s.p = null;     // must store a *i64 null, not a whole-struct null
+    s.n = 0;
+    return s;
+}
+main :: () -> i64 {
+    s := mk();
+    print("n {}\n", s.n);   // n 0
+    return 0;
+}

examples/types/expected/0193-types-sret-array-before-pointer.exit

@@ -0,0 +1 @@
+0

examples/types/expected/0193-types-sret-array-before-pointer.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0193-types-sret-array-before-pointer.stdout

@@ -0,0 +1 @@
+n 0