Runtime_DeclareLookupSlot is used when generating code for var and function declarations
originating in an eval. Over time, it's accumulated quite a bit of cruft, which this CL removes:

  - With legacy const gone, lookup slots never have any property attributes.
  - There was a bit signaling that the variable was from an eval, but that was redundant since
    DeclareLookupSlot is only used for eval.
  - Some Proxy-related code didn't make sense here.

Its name was also not terribly clear: while "LookupSlot" is used in several places, this
particular function is only used for declaring variables and functions inside sloppy eval.
Renamed (and split into two) to make this clear for future archeologists.

Also added various DCHECKs to check the assumptions being made.

Review-Url: https://codereview.chromium.org/2061173002
Cr-Commit-Position: refs/heads/master@{#37111}