[clang-tidy] Give readability-redundant-string-init a customizable list of string types to fix

Summary:
This patch adds a feature requested in https://reviews.llvm.org/D69238 to enable `readability-redundant-string-init` to take a list of strings to apply the fix to rather than hard-coding `basic_string`. It adds a `StringNames` option of semicolon-delimited names of string classes to which to apply this fix. Tests ensure this works with test class out::TestString as well as std::string and std::wstring as before. It should be applicable to llvm::StringRef, QString, etc.

Reviewers: MyDeveloperDay, aaron.ballman, hokein, alexfh, JonasToth, gribozavr2

Patch by: poelmanc

Subscribers: gribozavr2, xazax.hun, Eugene.Zelenko, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D69548