Move FormattersMatchCandidate flags to a struct.

This removes some error-prone repetition in
FormatManager::GetPossibleMatches, where the same three boolean flags
are passed in a row multiple times as arguments to recursive calls to
GetPossibleMatches.

Instead of:
```
  // same flags, but with did_strip_typedef set to true.
  GetPossibleMatches(..., did_strip_ptr, did_strip_ref, true);
```
we can now say
```
  GetPossibleMatches(..., current_flags.WithStrippedTypedef());
```
which hopefully makes the intent clearer, and more readable in case we
add another flag.

Reviewed by: DavidSpickett, labath

Differential Revision: https://reviews.llvm.org/D131459