[clang-tidy] readability-redundant-string-init now flags redundant...

[clang-tidy] readability-redundant-string-init now flags redundant initialisation in Field Decls and Constructor Initialisers

Summary:
The original behaviour of this check only looked at VarDecls with strings that had an empty string initializer. This has been improved to check for FieldDecls with an in class initializer as well as constructor initializers.

Addresses [[ https://bugs.llvm.org/show_bug.cgi?id=44474 | clang-tidy "modernize-use-default-member-init"/"readability-redundant-string-init" and redundant initializer of std::string ]]

Reviewers: aaron.ballman, alexfh, hokein

Reviewed By: aaron.ballman

Subscribers: merge_guards_bot, mgorny, Eugene.Zelenko, xazax.hun, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D72448