Refactor MethodDebugInfo (input of DWARF writer).

Do not pass CompiledMethod pointer through since it is only available
during AOT compile but not during JIT compile or at runtime. Creating
mock CompiledMethod just pass data is proving increasingly tricky, so
copy the fields that we need to MethodDebugInfo instead.

Change-Id: I820297b41e769fcac488c0ff2d2ea0492bb13ed8