[DebugInfo] Unify ChecksumKind and Checksum value in DIFile

Rather than encode the absence of a checksum with a Kind variant, instead put
both the kind and value in a struct and wrap it in an Optional.

Differential Revision: http://reviews.llvm.org/D43043



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324928 91177308-0d34-0410-b5e6-96231b3b80d8
17 files changed