[Support] Add support for unique_ptr<> to Casting.h.

[Support] Add support for unique_ptr<> to Casting.h.

Often you have a unique_ptr<T> where T supports LLVM's
casting methods, and you wish to cast it to a unique_ptr<U>.
Prior to this patch, this requires doing hacky things like:

unique_ptr<U> Casted;
if (isa<U>(Orig.get()))
  Casted.reset(cast<U>(Orig.release()));

This is overly verbose, and it would be nice to just be able
to use unique_ptr directly with cast and dyn_cast.  To this end,
this patch updates cast<> to work directly with unique_ptr<T>,
so you can now write:

auto Casted = cast<U>(std::move(Orig));

Since it's possible for dyn_cast<> to fail, however, we choose
to use a slightly different API here, because it's awkward to
write

if (auto Casted = dyn_cast<U>(std::move(Orig))) {}

when Orig may end up not having been moved at all.  So the
interface for dyn_cast is

if (auto Casted = unique_dyn_cast<U>(Orig)) {}

Where the inclusion of `unique` in the name of the cast operator
re-affirms that regardless of success of or fail of the casting,
exactly one of the input value and the return value will contain
a non-null result.

Differential Revision: https://reviews.llvm.org/D31890

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300098 91177308-0d34-0410-b5e6-96231b3b80d8