Fix Lint/ShadowedException to block arbitrary code execution

[pocke]

Problem

Lint/ShadowedException cop has an security risk.
An attacker can execute arbitrary code on a rubocop executing computer.

Reproduce

# test.rb
begin
  something
rescue puts('Arbitrary-code-execution!')
  handle_exception
rescue RuntimeError
  handle_standard_error
end

$ rubocop test.rb  --only Lint/ShadowedException --cache false
Inspecting 1 file
Arbitrary-code-execution!
.

1 file inspected, no offenses detected

Change

The cop evaluates const expressions only.
Other expressions(variables, method call, ....) are ignored.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[bbatsov]

This description is a bit vague.

[pocke]

@bbatsov Fixed the description

[bbatsov]

Hmm, the build is failing for Ruby 1.9.3. Seem that the json gem dropped support for 1.9.3...

[bbatsov]

The build is affected by #3278