Add performance-single-char-stream-insert check

[LegalizeAdulthood]

Bugzilla Link	26551
Version	unspecified
OS	All
CC	@EugeneZelenko

Extended Description

Add a performance check that looks for string literal insertion into an output stream where the string literal is a single character.

Replace the string literal insertion with a character literal insertion.

Special cases to consider:

str << "'"; => str << ''';
str << """ => str << '"';

[llvmbot]

Does it have any performance changes? I used to do this kind of optimizations, but
once I have heard that it doesn't make any difference with iostreams

[LegalizeAdulthood]

It's the difference between inserting a single character and looping over a single character string literal. I'm not saying the difference is huge, but it is always better to directly express that work not be done if it isn't necessary.

[LegalizeAdulthood]

This check is similar to performance-faster-string-find

[denzor200]

This check is similar to performance-faster-string-find

BTW, does anobody knows why only find? This idea also relevant for other std::string's operations:

std::string s("a");       // → std::string s(1, 'a');
s = "b";                  // → s = 'b';
s = std::string() + "text" + "a" + "more text"; // → s = std::string() + "text" + 'a' + "more text";
s += "a";                 // → s += 'a';
s.append("a");            // → s.append(1, 'a');
s.replace(pos, len, "a"); // → s.replace(pos, len, 1, 'a');

I haven't measured how does it affect performance in practise, but if it increases, why not to implement all of that in this new check?