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

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".

Differential Revision: https://reviews.llvm.org/D125059

clang/lib/Lex/LiteralSupport.cpp

@@ -320,10 +320,8 @@ 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");
-  Str.append(ResultBuf, ResultPtr);
+  if (llvm::ConvertCodePointToUTF8(Codepoint, ResultPtr))
+    Str.append(ResultBuf, ResultPtr);
 }
 
 void clang::expandUCNs(SmallVectorImpl<char> &Buf, StringRef Input) {

clang/test/Lexer/unicode.c

@@ -28,6 +28,9 @@ CHECK : The preprocessor should not complain about Unicode characters like ©.
 
         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;