[ADT] Add SmallVectorWithFlags with packed user flags

Structures that hold a SmallVector followed by a bool or a couple of
flag bits pay up to eight bytes of padding for those bits.
SmallVectorWithFlags stores one to four user flag bits in the high bits
of the capacity field, so it has the same size and inline capacity as
SmallVector.

Only capacity(), read by the growth check in push_back, masks the flags
out; begin(), end(), and size() are untouched. The cost is a maximum
capacity divided by 2^FlagBits, which with the 32-bit size type and
four flag bits is 268 million elements.

The SmallVector implementation layers take a FlagBits template
parameter, which plain SmallVector instantiates with zero bits so that
every mask folds away; its interface and codegen are unchanged. The
out-of-line growth routines stay in SmallVectorBase and remain
instantiated once per size type, with masked variants taking the
capacity mask at run time. The trade-off versus a composed
{ SmallVector, uint8_t Flags } is that the flagged container is not a
SmallVectorImpl<T>: use ArrayRef<T> for read-only interfaces and
SmallVectorWithFlagsImpl<T, FlagBits> for output parameters.

Element operations preserve flags; complete-container copy, move, and
swap include them. Equality compares flags and elements, and ordering
compares elements before flags. Comparison against ArrayRef is
intentionally not provided; convert explicitly for an element-only
comparison.

Assisted-by: Codex
Assisted-by: Claude Code