Fix handling of escaped backslashes in expanded string (#464) (#481)

Original fix uses std::copy_if algorithm with overlapping input and output
ranges - that behaviour is undefined.
After reviewing it with @Maiqel and @lukasz-matysiak we have replace it
with std::remove_if algorithm - which better describe what is happening
and does not excercise undefined behaviour.

From https://en.cppreference.com/w/cpp/algorithm/copy:

> The behavior is undefined if d_first is within the range [first, last)

Author: filipsajdak

source/lex.h

@@ -429,9 +429,9 @@ auto expand_string_literal(
             //  Then put interpolated chunk into ret
             auto chunk = std::string{text.substr(open, pos - open)};
             { // unescape chunk string
-                auto last_it = std::copy_if(std::begin(chunk), std::end(chunk), std::begin(chunk), [escape = false](const auto& e) mutable {
+                auto last_it = std::remove_if(std::begin(chunk), std::end(chunk), [escape = false](const auto& e) mutable {
                     escape = !escape && e == '\\';
-                    return !escape;
+                    return escape;
                 });
                 chunk.erase(last_it, std::end(chunk));
             }