UnsilencedDeprecation failures with namespace qualifiers

Problem/Motivation

When enforcing built-in functions to be fully qualified (See rationale https://github.com/previousnext/coding-standard/issues/80), the Drupal.Semantics.UnsilencedDeprecation.UnsilencedDeprecation rule incorrectly reports an error.

Steps to reproduce

@\trigger_error($message)

Error with:

All trigger_error calls used for deprecation must be prefixed by an "@"
| | (Drupal.Semantics.UnsilencedDeprecation.UnsilencedDeprecation)

Proposed resolution

Check for slashes or be more intelligent about getting the true function name. A backslash is after all not a part of the function name.

Even if the FQ rule is not endorsed by this project, it should be more true to how PHP looks at keywords/identifiers etc.

Remaining tasks

Fix