[clang][NFC] Specify Type and ExtQuals as having 16-byte alignment (#68377)

While working on LLDB visualizer for `QualType`, I stumbled upon `Type` and `ExtQuals` defined with `alignas(8)`. Such alignment leaves only 3 lower bits available for pointer tagging, whereas `QualType` requires 4 (3 qualifiers + discriminator between `Type *` and `ExtQuals *`). Turns out `Type` and its derived classes are allocated with `TypeAlignment == 16` passed to `Allocate()`. So I'm removing misleading `alignas(8)` and fixing corresponding static asserts. Since they are already allocated with 16-byte alignment, this is a non-functional change.