Detect NulInCStr error earlier.

By making it an `EscapeError` instead of a `LitError`. This makes it
like the other errors produced when checking string literals contents,
e.g. for invalid escape sequences or bare CR chars.

NOTE: this means these errors are issued earlier, before expansion,
which changes behaviour. It will be possible to move the check back to
the later point if desired. If that happens, it's likely that all the
string literal contents checks will be delayed together.

One nice thing about this: the old approach had some code in
`report_lit_error` to calculate the span of the nul char from a range.
This code used a hardwired `+2` to account for the `c"` at the start of
a C string literal, but this should have changed to a `+3` for raw C
string literals to account for the `cr"`, which meant that the caret in
`cr"` nul error messages was one short of where it should have been. The
new approach doesn't need any of this and avoids the off-by-one error.

crates/parser/src/lexed_str.rs

@@ -371,6 +371,7 @@ fn error_to_diagnostic_message(error: EscapeError, mode: Mode) -> &'static str {
             "non-ASCII character in byte string literal"
         }
         EscapeError::NonAsciiCharInByte => "non-ASCII character in raw byte string literal",
+        EscapeError::NulInCStr => "null character in C string literal",
         EscapeError::UnskippedWhitespaceWarning => "",
         EscapeError::MultipleSkippedLinesWarning => "",
     }

crates/syntax/src/validation.rs

@@ -106,6 +106,9 @@ fn rustc_unescape_error_to_string(err: unescape::EscapeError) -> (&'static str,
         EE::NonAsciiCharInByte  => {
             "Byte literals must not contain non-ASCII characters"
         }
+        EE::NulInCStr  => {
+            "C strings literals must not contain null characters"
+        }
         EE::UnskippedWhitespaceWarning => "Whitespace after this escape is not skipped",
         EE::MultipleSkippedLinesWarning => "Multiple lines are skipped by this escape",