suggest using raw string literals when invalid escapes appear

i'd guess about 70% of "bad escape" cases occur when someone meant to
use a raw string literal because they're passing it directly to
Regex::new(). this emits an advisory (Applicability::MaybeIncorrect)
help: suggestion to the user that they use an r"" string,
on top of the normal notes about looking at the
string literal documentation/spec.

compiler/rustc_parse/src/lexer/unescape_error_reporting.rs

@@ -185,6 +185,15 @@ pub(crate) fn emit_unescape_error(
                      version control settings",
                 );
             } else {
+                if !mode.is_bytes() {
+                    diag.span_suggestion(
+                        span_with_quotes,
+                        "if you meant to write a literal backslash (perhaps escaping in a regular expression), consider a raw string literal",
+                        format!("r\"{}\"", lit),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+
                 diag.help(
                     "for more information, visit \
                      <https://static.rust-lang.org/doc/master/reference.html#literals>",