Previously Collect() was used for types that implemented
CollectMemUsage() but otherwise Add() was used. This required the caller
to think about the type of the field and know/decide which method to
use.
Now, the caller always uses Collect() unless they are adding specific
byte values, in which case Add is used. Typically then, Add will only be
used to implement the CollectMemUsage() function.
To do this we require all Collect() methods to be templates so that they
all be a single overload set. The Collect on BumpPtrAllocator is
converted to a template that checks
`std::same_as<llvm::BumpPtrAllocator, T>`.