port 59546149 (r35632)

  original commit message:
  Now that all 'const' declarations are of the ES2015 variety, the only
  use of CONST_LEGACY is for function name bindings in sloppy mode
  named function expressions.

  This patch aims to delete all code meant to handle other cases, which
  mostly had to do with hole initialization/hole checks. Since function
  name bindings are initialized at entry to a function, it's impossible
  to ever observe one in an uninitialized state.

  To simplify the patch further, it removes the `IMPORT` VariableMode,
  as it's not likely to be needed (IMPORT is identical to CONST for
  the purpose of VariableMode).

BUG=

Review URL: https://codereview.chromium.org/1902943003

Cr-Commit-Position: refs/heads/master@{#35641}