lower: diagnose passing a by-ref loop capture where a value is expected

`for xs: (*m)` binds `m` to a `*T`. Passing it directly to a parameter
that wants `T` produced invalid IR that only LLVM's verifier caught, with
the opaque 'Call parameter type does not match function signature'. Detect
it at the call site and emit a clear error with a fix-it suggesting `m.*`.

Add example 215 + expected output as a regression test.

examples/215-ref-capture-value-arg-diagnostic.sx

@@ -0,0 +1,18 @@
+// A by-reference loop capture (`for xs: (*m)`) binds `m` to a `*T`.
+// Passing it where a `T` value is expected used to slip through to the
+// LLVM verifier ("Call parameter type does not match function signature").
+// The compiler now reports it at the call site with a fix-it: write `m.*`.
+
+#import "modules/std.sx";
+
+Move :: struct { flag: s64; }
+
+take :: (m: Move) -> s64 { return m.flag; }
+
+main :: () -> s32 {
+    moves : [2]Move = .[ Move.{ flag = 1 }, Move.{ flag = 2 } ];
+    for moves: (*m) {
+        take(m);
+    }
+    return 0;
+}