AsmPrinter: Cleanup DIEValue::EmitValue() API, NFC

Stop taking a `dwarf::Form` in `DIEValue::EmitValue()` and
`DIEValue::SizeOf()`, since they're always passed `DIEValue::getForm()`
anyway.  This is just left over from when `DIEValue` didn't know its own
form.

llvm-svn: 240566