[analyzer][Casting] Support isa, cast, dyn_cast of SVals

This change specializes the LLVM RTTI mechanism for SVals.
After this change, we can use the well-known `isa`, `cast`, `dyn_cast`.

Examples:

  // SVal V = ...;
  // Loc MyLoc = ...;

  bool IsInteresting = isa<loc::MemRegionVal, loc::GotoLabel>(MyLoc);
  auto MRV = cast<loc::MemRegionVal>(MyLoc);
  Optional<loc::MemRegionVal> MaybeMRV = dyn_cast<loc::MemRegionVal>(V)

The current `SVal::getAs` and `castAs` member functions are redundant at
this point, but I believe that they are still handy.

The member function version is terse and reads left-to-right, which IMO
is a great plus. However, we should probably add a variadic `isa` member
function version to have the same casting API in both cases.

Thanks for the extensive TMP help @bzcheeseman!

Reviewed By: bzcheeseman

Differential Revision: https://reviews.llvm.org/D125709

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