[clang-tidy] Extract string header from redundant-string-cstr checker

In preparation for using the implementation of basic_string and
basic_string_view from redundant-string-cstr.cpp in other checks, let's
extract it to a header.

Using the existing <string.h> to provide size_t, using that to define
size_type and using it rather than the previous "size" type makes it
easier to provide the size() method later and makes the implementation
closer to the standard.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D144216