[clang-tidy] Fix readability-identifer-naming Hungarian CString options

When reading readability-identifier-naming.HungarianNotation.CString
options, correctly use the type string stored in CStr.second instead of
the option name (CStr.first) as the HNOption.CString map key.

This will make CString options really working and properly parsed by the
checker.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D144431