Unify the return value of GetByteSize to an llvm::Optional<uint64_t> (NFC-ish)

This cleanup patch unifies all methods called GetByteSize() in the
ValueObject hierarchy to return an optional, like the methods in
CompilerType do. This means fewer magic 0 values, which could fix bugs
down the road in languages where types can have a size of zero, such
as Swift and C (but not C++).

Differential Revision: https://reviews.llvm.org/D84285