[libtooling][clang-tidy] Fix off-by-one rendering issue with SourceRanges

There was an off-by-one issue with calculating the *exact* end location
of token ranges (as given by SomeDecl->getSourceRange()) which resulted in:

  xxx(something)
      ^~~~~~~~   // Note the missing ~ under the last character.

In addition, a test is added to keep the behaviour in check in the future.

This patch hotfixes commit 3b677b81.