Scalable type size queries (clang)

[huntergr]

Summary

Initial changes for clang to use the scalable type size queries (https://reviews.llvm.org/D53137)

This isn't ready for inclusion yet, more discussion on the mailing list required.

[huntergr]

Abandoning this. At the devmeeting it was agreed that 'getPrimitiveSizeInBits' should continue to work as-is for fixed-length vectors and only behave differently for scalable vectors.