Fix `static_assert()` scope in `CombinedAllocator`.

It should be at the class scope and not inside the `Init(...)` function
because we want to error out as soon as the wrong type is constructed.
At the function scope the `static_assert` is only checked if the
function might be called.

This is a follow up to r349957.

rdar://problem/45284065

llvm-svn: 349960