[libtooling][clang-tidy] Fix diagnostics not highlighting fed SourceRanges

Fixes bug http://bugs.llvm.org/show_bug.cgi?id=49000.

This patch allows Clang-Tidy checks to do

    diag(X->getLocation(), "text") << Y->getSourceRange();

and get the highlight of `Y` as expected:

    warning: text [blah-blah]
        xxx(something)
        ^   ~~~~~~~~~

Reviewed-By: aaron.ballman, njames93

Differential Revision: http://reviews.llvm.org/D98635