Detect "index" variable names to avoid build issues

[lrstewart]

Problem:

An "index" function exists in strings.h, and older compilers (<GCC 4.8) warn if any local variables called "index" are used because they are considered to shadow that declaration. This issue broke our builds before #2540, and we almost introduced the same bug in #2589 (comment).

"index" is a reasonable variable name in other languages, and we'll probably make this mistake again. We should detect and prevent it with a linter.

Solution:

We can add a check for "index" to grep_simple_mistakes.sh, which runs against every pull request.