[clang] Don't warn on zero literals with -std=c2y (#149688)

Fixes #149669; the old check compared with the end of the literal, but
we can just check that after parsing digits, we're pointing to one
character past the token start.

(cherry picked from commit 8366dc2)

Author: anematode

clang/lib/Lex/LiteralSupport.cpp

@@ -1467,7 +1467,7 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
   if (s != PossibleNewDigitStart)
     DigitsBegin = PossibleNewDigitStart;
   else
-    IsSingleZero = (s == ThisTokEnd); // Is the only thing we've seen a 0?
+    IsSingleZero = (s == ThisTokBegin + 1);
 
   if (s == ThisTokEnd)
     return; // Done, simple octal number like 01234

clang/test/C/C2y/n3353.c

@@ -44,7 +44,12 @@ static const void *ptr = 0o0;  /* ext-warning {{octal integer literals are a C2y
 #endif
 
 // 0 by itself is not deprecated, of course.
-int k = 0;
+int k1                = 0;
+unsigned int k2       = 0u;
+long k3               = 0l;
+unsigned long k4      = 0ul;
+long long k5          = 0ll;
+unsigned long long k6 = 0ull;
 
 // Test a preprocessor use of 0 by itself, which is also not deprecated.
 #if 0
@@ -65,7 +70,6 @@ static_assert(__extension__ _Generic(typeof(l), const int : 1, default : 0)); //
 
 // Note that 0o by itself is an invalid literal.
 int m = 0o; /* expected-error {{invalid suffix 'o' on integer constant}}
-               c2y-warning {{octal literals without a '0o' prefix are deprecated}}
              */
 
 // Ensure negation works as expected.
@@ -83,13 +87,11 @@ int n = 0o18; /* expected-error {{invalid digit '8' in octal constant}}
                  cpp-warning {{octal integer literals are a Clang extension}}
                */
 int o1 = 0o8; /* expected-error {{invalid suffix 'o8' on integer constant}}
-                 c2y-warning {{octal literals without a '0o' prefix are deprecated}}
                */
 // FIXME: however, it matches the behavior for hex literals in terms of the
 // error reported. Unfortunately, we then go on to think 0 is an octal literal
 // without a prefix, which is again a bit confusing.
 int o2 = 0xG; /* expected-error {{invalid suffix 'xG' on integer constant}}
-                 c2y-warning {{octal literals without a '0o' prefix are deprecated}}
                */
 
 // Show that floating-point suffixes on octal literals are rejected.
@@ -130,7 +132,6 @@ constexpr int p = 0o0'1'2'3'4'5'6'7; /* compat-warning {{octal integer literals
                                       */
 static_assert(p == 01234567); // c2y-warning {{octal literals without a '0o' prefix are deprecated}}
 int q = 0o'0'1; /* expected-error {{invalid suffix 'o'0'1' on integer constant}}
-                   c2y-warning {{octal literals without a '0o' prefix are deprecated}}
                  */
 
 #define M 0o123