Add a new check, readability-simplify-subscript-expr, that diagnoses array...

Add a new check, readability-simplify-subscript-expr, that diagnoses array subscript expressions that can be simplified.

Currently, diagnoses code that calls container.data()[some_index] when the container exposes a suitable operator[]() method that can be used directly.

Patch by Shuai Wang.

llvm-svn: 332519