LOAD-TIME-VALUE improvements

LOAD-TIME-VALUE improvements

 * Derive the type of :TOPLEVEL lambdas properly. Without this we were
   never able to utilize the type from COMPILE-LOAD-TIME-STUFF.

 * Use the source-type whenever it is more accurate than the type from
   COMPILE-LOAD-TIME-STUFF -- eg. when using a value cell.

 * Add ALIAS argument to IR1-CONVERT, allowing saving alternate forms
   into *CURRENT-PATH*: this allows

     (defparameter *var* 10)

     (compile nil '(lambda () (the list (load-time-value *var*))))

    to give the warning

      ;   Derived type of *VAR* is
      ;     (VALUES (INTEGER 10 10) &OPTIONAL),
      ;   conflicting with its asserted type
      ;     LIST.

    instead of the much less useful

      ;   Constant 10 conflicts with its asserted type LIST.

  * Use THE-IN-POLICY directly in LOAD-TIME-VALUE, allowing the file-
    compiler to report the LOAD-TIME-VALUE form for type-conflicts
    instead of (TRULY-THE <type> (%LOAD-TIME-VALUE ...))