[clang-tidy] new check 'readability-isolate-declaration'

Summary:
This patch introduces a new clang-tidy check that matches on all `declStmt` that declare more then one variable
and transform them into one statement per declaration if possible.

It currently only focusses on variable declarations but should be extended to cover more kinds of declarations in the future.
It is related to https://reviews.llvm.org/D27621 and does use it's extensive test-suite. Thank you to firolino for his work!

Reviewers: rsmith, aaron.ballman, alexfh, hokein, kbobyrev

Reviewed By: aaron.ballman

Subscribers: ZaMaZaN4iK, mgehre, nemanjai, kbarton, lebedev.ri, Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D51949

llvm-svn: 345735