Remove flawed diagnostic mapping functions

These haven't been usable since the early return was accidentally removed some
years ago causing all cases to fall through to the !Enabled condition.
ARCMigrate: Introduce proper diagnostics for TransformActions

This starts to switch ARCMT to use proper diagnostic messages. The old use was
based on incorrect example code from the documentation.

The logic of the previous report() functions has been retained to support any
external consumers that might be intercepting diagnostic messages through the
old interface.

Note that the change in test/Misc/warning-flags.c isn't a new warning without a
flag, rather one that was previously invisible to the test. Adding a flag might
be a good idea though.