[AST] clang::VectorType supports any size (that fits in unsigned)

Summary:
This matches llvm::VectorType.
It moves the size from the type bitfield into VectorType, increasing size by 8
bytes (including padding of 4). This is OK as we don't expect to create terribly
many of these types.

c.f. D77313 which enables large power-of-two sizes without growing VectorType.

Reviewers: efriedma, hokein

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77335