[NFC, Refactor] Modernize enum FunctionDefinitionKind (DeclSpech.h) into a scoped enum

Reviewed by aaron.ballman, rsmith, wchilders
Highlights of review:
- avoid specifying an underlying type (unless such an enum is stored (or part of an abi?))
- avoid using enums as bit-fields, preferring unsigned bit-fields that we static_cast enumerators to. (MS's abi laysout enum bit-fields differently).
- clang-format, clang-format, clang-format.

https://reviews.llvm.org/D91035

Thank you!