[DebugInfo] Fix misleading using of DWARF forms with DIELabel. NFCI.

DIELabel can emit only 32- or 64-bit values, while it was created in
some places with DW_FORM_udata, which implies emitting uleb128.
Nevertheless, these places also expected to emit U32 or U64, but just
used a misleading DWARF form. The patch updates those places to use more
appropriate DWARF forms and restricts DIELabel::SizeOf() to accept only
forms that are actually used in the LLVM codebase.

Differential Revision: https://reviews.llvm.org/D84094