This avoid lots of converting back and forth between the two types.

In particular, this makes life simpler when we want to use a general
GHC SDoc in the middle of some LLVM.

It was equivalent to showPpr anyway.

We also store the Doc still, although we don't have to; we could
just convert back again, at the cost of some inefficiency.

But we do need the String, so that we can print it in the Show instance.

To help with this, we now also pass DynFlags around inside the SpecM
monad.

This shouldn't be any slower; the FilePaths were already going via
String so that we could normalise them.

This means that very early on, when printing flag parsing errors, we
can do so without having a DynFlags yet.

A side-effect is that we can no longer use the LogAction in
defaultErrorHandler, as we don't have DynFlags at that point.
But all that defaultErrorHandler did is to print Strings as
SevFatal, so now it takes a 'FatalMessager' instead.

We don't use it yet

We now include the String and the SDoc in the exception, and don't
flatten them into a String until near the top-level