DiagnosticIds: Fix offset/ID calculation, no impact outside this code.

Patch by Will Dietz:

Minor touchup so the values of Offset/ID reflect their intention.
Previously, the sum (Offset+ID) was correct, but Offset/ID
individually were wrong.

Caught by investigating unsigned overflow reported by -fsanitize=integer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171421 91177308-0d34-0410-b5e6-96231b3b80d8