Thread safety analysis: Examine constructor arguments

Summary:
Instead of only examining call arguments, we also examine constructor
arguments applying the same rules.

That was an opportunity for refactoring the examination procedure to
work with iterators instead of integer indices. For the case of
CallExprs no functional change is intended.

Reviewers: aaron.ballman, delesley

Reviewed By: delesley

Subscribers: JonasToth, cfe-commits

Differential Revision: https://reviews.llvm.org/D52443

llvm-svn: 343831