[clang-tidy] Fix a crash in modernize-loop-convert around conversion operators

modernize-loop-convert checks and fixes when a loop that iterates over the
elements of a container can be rewritten from a for(...; ...; ...) style into
the "new" C++11 for-range format. For that, it needs to parse the elements of
that loop, like its init-statement, such as ItType it = cont.begin().
modernize-loop-convert checks whether the loop variable is initialized by a
begin() member function.

When an iterator is initialized with a conversion operator (e.g. for
(const_iterator it = non_const_container.begin(); ...), attempts to retrieve the
name of the initializer expression resulted in an assert, as conversion
operators don't have a valid IdentifierInfo.

I fixed this by making digThroughConstructors dig through conversion operators
as well.

Differential Revision: https://reviews.llvm.org/D113201