[DebugInfo] Make DIEDelta::SizeOf() more explicit. NFCI.

The patch restricts DIEDelta::SizeOf() to accept only DWARF forms that
are actually used in the LLVM codebase. This should make the use of the
class more explicit and help to avoid issues similar to fixed in D83958
and D84094.

Differential Revision: https://reviews.llvm.org/D84095