RFC: fix std::unique_ptr pretty-printer

[Tom Tromey]

>>>>> "Jonathan" == Jonathan Wakely <jwakely.gcc@gmail.com> writes:

>> $11 = std::unique_ptr containing (datum *) 0x6067d0

Jonathan> It's inconsistent with the other printers in that it prints
Jonathan> the stored type, unlike e.g. std::vector<int> which just says
Jonathan> "std::vector of length ..." but I think that's an improvement.

Yeah... without this bit it was just printing

$11 = std::unique_ptr containing 0x6067d0

Ordinarily, gdb will print the type here; but it doesn't when called
from Python.  I thought the typical output was easier to read.

Jonathan> Personally I'd prefer the element_type as part of the type, e.g.
Jonathan> "std::unique_ptr<datum> = 0x6067d0" but that would be even more
Jonathan> inconsistent!

I can make that change if you'd prefer.
I don't know why, but I didn't even think of it.

Tom