[Clang] Fix a crash when encountering an ill-formed delimited UCN.

[cor3ntin]

Summary

\u<DIGIT>{...} was incorrectly parsed as a valid UCN instead
of emitting a diagnostic, causing an assertion failure.

[cor3ntin]

@tahonermann gentle ping :)

[tahonermann]

Looks good! Sorry, Corentin, this review had fallen off my radar!

[cor3ntin]

@tahonermann Thanks :)

clang/lib/Lex/Lexer.cpp

llvm::Optional<uint32_t> Lexer::tryReadNumericUCN(const char *&StartPtr,
   }
 
   const char *CurPtr = StartPtr + CharSize;
   const char *KindLoc = &CurPtr[-1];
 
   uint32_t CodePoint = 0;
   while (Count != NumHexDigits || Delimited) {
     char C = getCharAndSize(CurPtr, CharSize);
-    if (!Delimited && C == '{') {
+    if (!Delimited && Count == 0 && C == '{') {
       Delimited = true;
       CurPtr += CharSize;
       continue;
     }
 
     if (Delimited && C == '}') {
       CurPtr += CharSize;
       FoundEndDelimiter = true;

clang/test/Preprocessor/ucn-pp-identifier.c

 #define capital_u_\U00FC
 // expected-warning@-1 {{incomplete universal character name}} expected-note@-1 {{did you mean to use '\u'?}} expected-warning@-1 {{whitespace}}
 // CHECK: note: did you mean to use '\u'?
 // CHECK-NEXT:   #define capital_u_\U00FC
 // CHECK-NEXT: {{^                   \^}}
 // CHECK-NEXT: {{^                   u}}
 
 #define \u{}           // expected-warning {{empty delimited universal character name; treating as '\' 'u' '{' '}'}} expected-error {{macro name must be an identifier}}
+#define \u1{123}       // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}} expected-error {{macro name must be an identifier}}
 #define \u{123456789}  // expected-error {{hex escape sequence out of range}} expected-error {{macro name must be an identifier}}
 #define \u{            // expected-warning {{incomplete delimited universal character name; treating as '\' 'u' '{' identifier}} expected-error {{macro name must be an identifier}}
 #define \u{fgh}        // expected-warning {{incomplete delimited universal character name; treating as '\' 'u' '{' identifier}} expected-error {{macro name must be an identifier}}
 #define \N{
 // expected-warning@-1 {{incomplete delimited universal character name; treating as '\' 'N' '{' identifier}}
 // expected-error@-2 {{macro name must be an identifier}}
 #define \N{}           // expected-warning {{empty delimited universal character name; treating as '\' 'N' '{' '}'}} expected-error {{macro name must be an identifier}}
 #define \N{NOTATHING}  // expected-error {{'NOTATHING' is not a valid Unicode character name}} \