AstGen: fix src loc for invalid coercions in tuple literals

test/cases/compile_errors/invalid_coercion_in_aggregate_literal.zig

@@ -0,0 +1,22 @@
+export fn invalidArrayElem() u8 {
+    const array_literal = [1]u8{@as(u8, 256)};
+    return array_literal[0];
+}
+
+export fn invalidTupleElem() u8 {
+    const tuple_literal = struct { u8 }{@as(u8, 256)};
+    return tuple_literal[0];
+}
+
+export fn invalidStructField() u8 {
+    const struct_literal = struct { field: u8 }{ .field = @as(u8, 256) };
+    return struct_literal.field;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:41: error: type 'u8' cannot represent integer value '256'
+// :7:49: error: type 'u8' cannot represent integer value '256'
+// :12:67: error: type 'u8' cannot represent integer value '256'