[NFC, Refactor] Modernize the TypeSpecifierWidth enum (Specifiers.h) to a scoped enum

Reviewed here: https://reviews.llvm.org/D91409 by Aaron.
Highlights of the review:
  - avoid an underlying type for enums
  - avoid enum bit fields (MSVC packing anomalies) and favor static_casts to unsigned bit-fields

Patch by Thorsten Schuett <schuett@gmail.com> w some minor fixes in SemaType.cpp where a couple asserts had to be repaired to deal with lack of implicit coversion to int.

Thanks Thorsten!