[clang-tidy] Correctly classify constant arrays and constant strings as...

[clang-tidy] Correctly classify constant arrays and constant strings as constants when checking identifiers naming

Summary:
They are not locally const qualified so they weren't classified as
constants by the readability-identifier-naming check.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: klimek, cfe-commits, xazax.hun

Patch by Beren Minor!

Differential Revision: https://reviews.llvm.org/D39363

llvm-svn: 320406