Merge from Microsoft 2021-09-08

[mattmccutchen-cci]

The important changes we want to start using or testing against:

• Support for checking calls to printf-like functions (Support variadic function calls in checked scope checkedc/checkedc-clang#1174). As Aaron said in yesterday's meeting, we should now change 3c -addcr not to treat those calls as unchecked, but we can do that later since the inconsistency won't actually break anything; I filed Update -addcr to allow printf-like function calls in checked regions #701 for follow-up.

• Bounds enforcement for function return values (Check return value bounds checkedc/checkedc-clang#1150), except when returning an unchecked pointer and the return type has an itype (per our discussion).

• Don't enforce bounds when assigning an unchecked pointer to a variable or struct field that has an itype rather than being fully checked (Bounds checking bounds-safe interfaces in unchecked scopes checkedc/checkedc-clang#1169). If I understand correctly that the problem John saw in our porting workflow was that accepting headers converted with -itypes-for-extern introduced compile errors in other files that assigned unchecked pointers to struct fields with itypes, this should fix that. For example, the following code no longer produces a compile error on the s.p = x:

  struct my_struct {
    char *p : count(1);
  };
  
  void assign_struct(void) {
    struct my_struct s;
    char *x;
    s.p = x;
  }

The 3C regression tests pass. Do we care about running any other tests now, or can we just update them later as needed?

[john-h-kastner]

Looks good. I think the bounds checking changes should help with our porting process.