[Lex] Don't assert when decoding invalid UCNs.

[sammccall]

Summary

Currently if a lexically-valid UCN encodes an invalid codepoint, then we
diagnose that, and then hit an assertion while trying to decode it.

Since there isn't anything preventing us reaching this state, remove the
assertion. expandUCNs("X\UAAAAAAAAY") will produce "XY".

[sammccall]

With this and D125049, I no longer get any low-hanging fuzzer crashes.

(Though we're probably mostly fuzzing the lexer. Providing a list of tokens as a dictionaly may help)

[hokein]

clang-tools-extra/pseudo/test/crash/bad-ucn.c

4 ↗

(On Diff #427494)

nit: this test seems duplicated with the one in unicode.c, I'd just remove it.

clang/lib/Lex/LiteralSupport.cpp

static unsigned ProcessCharEscape(const char *ThisTokBegin,
 
   return ResultChar;
 }
 
 static void appendCodePoint(unsigned Codepoint,
                             llvm::SmallVectorImpl<char> &Str) {
   char ResultBuf[4];
   char *ResultPtr = ResultBuf;
-  bool Res = llvm::ConvertCodePointToUTF8(Codepoint, ResultPtr);
-  (void)Res;
-  assert(Res && "Unexpected conversion failure");
+  if (llvm::ConvertCodePointToUTF8(Codepoint, ResultPtr))
     Str.append(ResultBuf, ResultPtr);
 }
 
 void clang::expandUCNs(SmallVectorImpl<char> &Buf, StringRef Input) {
   for (StringRef::iterator I = Input.begin(), E = Input.end(); I != E; ++I) {
     if (*I != '\\') {
       Buf.push_back(*I);
       continue;
     }

clang/test/Lexer/unicode.c

 #if PP_ONLY
 COPYRIGHT
 // CHECK: Copyright © {{2012}}
 CHECK : The preprocessor should not complain about Unicode characters like ©.
 #endif
 
         int _;
 
+extern int X\UAAAAAAAA; // expected-error {{not allowed in an identifier}}
+int Y = '\UAAAAAAAA'; // expected-error {{invalid universal character}}
+
 #ifdef __cplusplus
 
 extern int ༀ;
 extern int 𑩐;
 extern int 𐠈;
 extern int ꙮ;
 extern int  \u1B4C;     // BALINESE LETTER ARCHAIC JNYA - Added in Unicode 14
 extern int  \U00016AA2; // TANGSA LETTER GA - Added in Unicode 14