[BUG] Escaping '"' in string in interpolation fails assertion

[JohelEGP]

Title: Escaping '"' in string in interpolation fails assertion.

Minimal reproducer (https://cpp2.godbolt.org/z/vYsh34z5K):

main: () = {
  std::cout << "\"\"" << '\n'; // Prints `""`.
  std::cout << $R"("")" << '\n'; // Prints `""`.
  std::cout << "\"(\"\")$\"" << '\n'; // Prints `""`.
  std::cout << "(\"\\\"\\\"\")$" << '\n'; // cppfront asserts.
}

Commands:

cppfront -clean-cpp1 main.cpp2

Expected result:

A diagnostic, or to print "" like the other lines.

Actual result and error:

cppfront: source/lex.h:437: std::string cpp2::expand_string_literal(std::string_view, std::vector<error_entry>&, source_position): Assertion `pos == length-1 && text[pos] == '"'' failed.

[hsutter]

@filipsajdak I think you looked at this code last -- can you take a look at this issue?

[filipsajdak]

I am on it.