[MC] Minor cleanup to MCFixup::Kind handling. NFC.

[MC] Minor cleanup to MCFixup::Kind handling. NFC.

Prefer `MCFixupKind` where possible and add getTargetKind() to
convert to `unsigned` when needed rather than scattering cast
operators around the place.

Differential Revision: https://reviews.llvm.org/D59890

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369720 91177308-0d34-0410-b5e6-96231b3b80d8