Improve diagnostic mapping terminology

Diagnostic mappings are used to calculate the final severity of diagnostic
instances.

Detangle the implementation to reflect the terminology used in documentation
and bindings.

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210518 91177308-0d34-0410-b5e6-96231b3b80d8