Make SmallPtrSet count and find able to take const PtrType's

Summary:
For our set/map types, count/find normally take const references.
This works well for non-pointer types, but can suck for pointer
types.

DenseSet<int *> foo;
const int *b = nullptr;
foo.count(b) does not work

but the equivalent reference version does work
(patch to fix DenseSet/DenseMap coming up)
For SmallPtrSet, you have no such option.

The following will not work right now:
SmallPtrSet<int *> foo;
const int *b = nullptr;
foo.count(b);

This makes const correctness hard in some cases.
Example:
SmallPtrSet<Instruction *> InstructionsToErase;

You can't make this SmallPtrSet<const Instruction *> because then you
can't erase the instruction.  If I want to see if something is in the
set, I may only have a const Instruction *.  Given that count and find
are non-mutating, this should just work.

The places in our code base that do this resort to const_cast :(.

This patch makes count and find able to be used with const Instruction
* in the above SmallPtrSet examples.

This is a bit annoying because of where C++ applies the const, so we
have to remove the pointer type from the passed-in-type and rebuild it
with const.

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30608

llvm-svn: 297180