The new logic ensures that the error messages are the same in the
"import { <reserved word> }" and "import { foo as <reserved ord> }"
cases.
Also prepares ParseImportNames for returning both the import and local
names to ParseImportClause.
BUG=v8:1569
LOG=n
Review URL: https://codereview.chromium.org/952863006
Cr-Commit-Position: refs/heads/master@{#26863}
