Use dedicated string attribute for string literals

[Lancern]

Currently C/C++ string literals are lowered to a #cir.const_array with a built-in mlir::StringAttr that holds the constant string value. However, the built-in mlir::StringAttr does not hold the character set of the string literal (i.e. only char strings and char8_t strings are supported now).

We can introduce a new attribute #cir.string that holds the string literal value together with its character set information (char and char8_t, wchar_t, char16_t, char32_t). Based on the new attribute we can support wide string literals (wchar_t string), UTF-16 string literals (char16_t string), and UTF-32 string literals (char32_t string).

I'd love to help draft a PR for this if this is necessary.