[libTooling] Add function to determine associated text of a declaration.

Summary:
Second attempt -- the first was reverted in commit 0e480b39, because of test breakages. This revision fixes the cause of the test breakages.

Original description follows:
This patch adds `getAssociatedRange` which, for a given decl, computes preceding
and trailing text that would conceptually be associated with the decl by the
reader. This includes comments, whitespace, and separators like ';'.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72153