Add ReturnValueInfo class for storing function return information.
Refactoring to handle multiple return values.

- Replace use of address + CpuState pair in Thread,
  SpecificImageDebugInfo::CreateStackTrace() with a ReturnValueInfoList.
  Adjust all implementing subclasses and callers accordingly.

- DwarfImageDebugInfo::CreateReturnValue() -> CreateReturnValues().
  Now processes a list of return value information structures rather
  than just a single one. This means we can now handle multiple return
  values in a single statement. This still isn't entirely correct though,
  since, e.g. for functions whose return types fit in a register we need
  to either retrieve them immediately after function return, or store the
  CPU state at that point in time for later use in value retrieval,
  otherwise the return values will all be those of the last called function.
Remove no longer needed member.