Fix line table regression

Fix line table regression

Simon pointed out a line table regression, and after a couple of false
starts, I was able to reproduce it by hand using his instructions.

The bug is that most of the code in do_mixed_source_and_assembly uses
unrelocated addresses, but one spot does:

  pc = low;

... after the text offset has been removed.

This patch fixes the problem by introducing a new type to represent
unrelocated addresses in the line table.  This prevents this sort of
bug to some degree (it's still possible to manipulate a CORE_ADDR in a
bad way, this is unavoidable).

However, this did let the compiler flag a few spots in that function,
and now it's not possible to compare an unrelocated address from a
line table with an ordinary CORE_ADDR.

Regression tested on x86-64 Fedora 36, though note this setup never
reproduced the bug in the first place.  I also tested it by hand on
the disasm-optim test program.