[clang][NFC] In `CFGStmtMap`, do not use a `void *` data member, just use the object directly (#172528)

There is no reason to dynamically allocate `llvm::DenseMap` and try to
hide the type. A header we include anyway already includes `DenseMap.h`
so we save almost no compilation time. This change improves performance
by avoiding the dynamic allocation, and simplifies the code
considerably.

Now that we just have a regular data member, there is also no need for a
manual destructor, and the copy / move operations will do the right
thing.

In `getBlock`, we have some code that a comment claims is implementing
memoization, but in reality it does nothing. The relevant expression is
a conditional `(*SM)[X] = B`, but `B` is equal to `SM->find(X)->second`.

In `Accumulate`, we have a bunch of code to add things to the map for
the initial set-up. However, the original code would either find or
default construct an element, and then if the found element is equal to
the default constructed element it would set it to `B`. Rather than
doing this in two steps, we can simply use `try_emplace` to insert if
it's not already present. This change is sound only if the new element
we are inserting cannot be equal to the default constructed element, but
the element type is a pointer and this entire section of code assumes
`B` is not null.