Merge diagnostic group tables to reduce data size and relocation entries.

The individual group and subgroups tables are now two large tables. The option table stores an index into these two tables instead of pointers. This reduces the size of the options tabe since it doesn't need to store pointers. It also reduces the number of relocations needed.

My build shows this reducing DiagnosticsIDs.o and the clang binary by ~20.5K. It also removes ~400 relocation entries from DiagnosticIDs.o.