Fix off-by-one issue found by post-commit review

Author: AaronBallman

clang/lib/Lex/Lexer.cpp

@@ -2262,7 +2262,6 @@ bool Lexer::LexRawStringLiteral(Token &Result, const char *CurPtr,
   unsigned PrefixLen = 0;
 
   while (PrefixLen != 16 && isRawStringDelimBody(CurPtr[PrefixLen])) {
-    ++PrefixLen;
     if (!isLexingRawMode() &&
         llvm::is_contained({'$', '@', '`'}, CurPtr[PrefixLen])) {
       const char *Pos = &CurPtr[PrefixLen];
@@ -2271,6 +2270,7 @@ bool Lexer::LexRawStringLiteral(Token &Result, const char *CurPtr,
                     : diag::ext_cxx26_raw_string_literal_character_set)
           << StringRef(Pos, 1);
     }
+    ++PrefixLen;
   }
 
   // If the last character was not a '(', then we didn't lex a valid delimiter.

clang/test/Lexer/cxx2c-raw-strings.cpp

@@ -21,4 +21,8 @@ int main() {
   (void) R"\()\";
   // expected-error@-1 {{invalid character '\' in raw string delimiter}}
   // expected-error@-2 {{expected expression}}
+
+  (void) R"@(foo)@";
+  // cxx26-warning@-1 {{'@' in a raw string literal delimiter is incompatible with standards before C++2c}}
+  // precxx26-warning@-2 {{'@' in a raw string literal delimiter is a C++2c extension}}
 }