cs and ffi/unsafe/com: repair com-invoke for method without type

The argument type `(_ptr i _string/utf-16)` is not used on CS, because
a pointer to non-atomic memory cannot usefully be passed to a foreign
function from Racket CS.

Closes #3820

racket/collects/ffi/unsafe/com.rkt

@@ -333,7 +333,7 @@
    [GetTypeInfo (_hmfun _UINT _LCID (p : (_ptr o _pointer))
                         -> GetTypeInfo (cast p _pointer _ITypeInfo-pointer))
                 #:release-with-function Release]
-   [GetIDsOfNames (_hmfun _REFIID (_ptr i _string/utf-16)
+   [GetIDsOfNames (_hmfun _REFIID _ptr-to-string/utf-16
                           (_UINT = 1) _LCID
                           (p : (_ptr o _DISPID))
                           -> GetIDsOfNames

racket/collects/ffi/unsafe/private/win32.rkt

@@ -371,12 +371,29 @@
            2
            1))))
 
-(define (string->pointer s)
+(define (string->pointer s [alloc SysAllocStringLen])
   (let ([v (malloc _gcpointer)])
     (ptr-set! v _string/utf-16 s)
     (let ([p (ptr-ref v _gcpointer)])
       (let ([len (utf-16-length s)])
-        (SysAllocStringLen p len)))))
+        (alloc p len)))))
+
+;; Like (_ptr i _string/utf-16), but as a single atomic (and immobile
+;; and GCable) object, so it can be passed to a function in CS:
+(define _ptr-to-string/utf-16
+  (make-ctype _gcpointer
+              (lambda (s)
+                (string->pointer
+                 s
+                 (lambda (p len)
+                   (define slen (+ len 2)) ; add terminator
+                   (define p2 (malloc 'atomic-interior
+                                      (+ slen (ctype-sizeof _pointer))))
+                   (memcpy p2 (ctype-sizeof _pointer) p slen)
+                   (ptr-set! p2 _pointer 0 p)
+                   p2)))
+              (lambda (p)
+                (cast p _pointer _string/utf-16))))
 
 (define _SAFEARRAY-pointer (_cpointer 'SAFEARRAY))