[clang-tidy] Support begin/end free functions in modernize-loop-convert

The modernize-loop-convert check will now match iterator based loops
that call the free functions 'begin'/'end', as well as matching the
free function 'size' on containers.

Test plan: Added unit test cases matching free function calls on
containers, and a single negative test case for length() which is not
supported.

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D140760