Update: fix false negative of no-useless-escape in template literal tags

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix

Tell us about your environment

• ESLint Version: master
• Node Version: 7.7.2
• npm Version: 4.1.2

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

rules:
  no-useless-escape: error
parserOptions:
  ecmaVersion: 2015

What did you do? Please include the actual source code causing the issue.

`\a```

What did you expect to happen?

I expected the \a to be reported as a useless escape.

What actually happened? Please include the actual, raw output from ESLint.

No errors were reported.

What changes did you make? (Give an overview)

This fixes a check in no-useless-escape for tagged template literals. Previously, the rule would exclude any TemplateLiteral whose parent was a TaggedTemplateExpression. This is because backslashes in the "literal" part of a tagged template can be observed by the tag function. However, this doesn't apply to the tag itself, so if a template literal is a tag for another template literal, the rule should check for useless escapes in the tag.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kaicataldo, @onurtemizkan and @vitorbal to be potential reviewers.

[eslintbot]

LGTM