bad error message on incorrect string literal #18079 (#81670)

(bad error message on incorrect string literal)

Fixed the error message for incorrect string literal

before:

```
test.cpp:1:19: error: invalid character '
' character in raw string delimiter; use PREFIX( )PREFIX to delimit raw string
char const* a = R"
                  ^
```

now:

```
test.cpp:1:19: error: invalid newline character in raw string delimiter; use PREFIX( )PREFIX to delimit raw string
    1 | char const* a = R"
      |                   ^
```

---------

Co-authored-by: Jon Roelofs <jroelofs@gmail.com>

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -100,7 +100,10 @@ def err_raw_delim_too_long : Error<
   "raw string delimiter longer than 16 characters"
   "; use PREFIX( )PREFIX to delimit raw string">;
 def err_invalid_char_raw_delim : Error<
-  "invalid character '%0' character in raw string delimiter"
+  "invalid character '%0' in raw string delimiter"
+  "; use PREFIX( )PREFIX to delimit raw string">;
+def err_invalid_newline_raw_delim : Error<
+  "invalid newline character in raw string delimiter"
   "; use PREFIX( )PREFIX to delimit raw string">;
 def err_unterminated_raw_string : Error<
   "raw string missing terminating delimiter )%0\"">;

clang/lib/Lex/Lexer.cpp

@@ -2270,6 +2270,8 @@ bool Lexer::LexRawStringLiteral(Token &Result, const char *CurPtr,
       const char *PrefixEnd = &CurPtr[PrefixLen];
       if (PrefixLen == 16) {
         Diag(PrefixEnd, diag::err_raw_delim_too_long);
+      } else if (*PrefixEnd == '\n') {
+        Diag(PrefixEnd, diag::err_invalid_newline_raw_delim);
       } else {
         Diag(PrefixEnd, diag::err_invalid_char_raw_delim)
           << StringRef(PrefixEnd, 1);

clang/test/Lexer/raw-string-dlim-invalid.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -E -fsyntax-only -verify %s
+
+// expected-error@+2{{invalid character ')' in raw string delimiter; use PREFIX( )PREFIX to delimit raw string}}
+// expected-error@+1{{expected expression}}
+char const *str1 = R")";
+
+// expected-error@+2{{invalid newline character in raw string delimiter; use PREFIX( )PREFIX to delimit raw string}}
+// expected-error@+1{{expected expression}}
+char const* str2 = R"";