[clang-tidy] Improve hicpp-exception-baseclass to handle generic code better

Summary:
This patch is a followup to the first revision D36583, that had problems with 
generic code and its diagnostic messages, which were found by @lebedev.ri

Reviewers: alexfh, aaron.ballman, lebedev.ri, hokein

Reviewed By: aaron.ballman, lebedev.ri

Subscribers: klimek, sbenza, cfe-commits, JDevlieghere, lebedev.ri, xazax.hun

Differential Revision: https://reviews.llvm.org/D37060

llvm-svn: 312134