Handle unterminated raw strings with no #s properly

The modified code to handle parsing raw strings didn't properly account for the case where there was no "#" on either end and erroneously reported this strings as complete. This lead to a panic trying to read off the end of the file.

src/test/ui/parser/raw/issue-70677-panic-on-unterminated-raw-str-at-eof.rs

@@ -0,0 +1,5 @@
+// This won't actually panic because of the error comment -- the `"` needs to be
+// the last byte in the file (including not having a trailing newline)
+// Prior to the fix you get the error: 'expected item, found `r" ...`'
+// because the string being unterminated wasn't properly detected.
+r" //~ unterminated raw string

src/test/ui/parser/raw/issue-70677-panic-on-unterminated-raw-str-at-eof.stderr

@@ -0,0 +1,9 @@
+error[E0748]: unterminated raw string
+  --> $DIR/issue-70677-panic-on-unterminated-raw-str-at-eof.rs:5:1
+   |
+LL | r"
+   | ^ unterminated raw string
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0748`.