[clang-tidy] Fix bugprone-argument-comment-check to correctly ignore implicit constructors.

Summary:
After revision 370919, this check incorrectly flags certain cases of implicit
constructors. Specifically, if an argument is annotated with an
argument-comment and the argument expression triggers an implicit constructor,
then the argument comment is associated with argument of the implicit
constructor.

However, this only happens when the constructor has more than one argument.
This revision fixes the check for implicit constructors and adds a regression
test for this case.

Note: r370919 didn't cause this bug, it simply uncovered it by fixing another
bug that was masking the behavior.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67744

llvm-svn: 372317