Port 59546149

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).

R=adamk@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1902163003

Cr-Commit-Position: refs/heads/master@{#35635}