[AST][SVE] Add new Type queries for sizeless types

One of the defining features of the SVE ACLE types is that they
are "sizeless"; see the SVE ACLE spec:

    https://developer.arm.com/docs/100987/0000/arm-c-language-extensions-for-sve

or the email message:

    http://lists.llvm.org/pipermail/cfe-dev/2019-June/062523.html

for a fuller definition of what that means.

This patch adds two associated type queries:

- isSizelessBuiltinType asks specifically about types that are built
  into clang.  It is effectively an enum range check.

- isSizelessType instead tests for any type that has the "sizeless" type
  property.  At the moment it only returns true for the built-in types,
  but it seems better not to hard-code that assumption throughout
  the codebase.  (E.g. we could in principle support some form of
  user-defined sizeless types in future.  Even if that seems unlikely
  and never actually happens, the possibility at least exists.)

Differential Revision: https://reviews.llvm.org/D75570