[clang] Fix assertion failure in injectEmbedTokens (#99624)

Summary:
It seems for C++ lexer has some caching ability which doesn't expect
injecting "new" tokens in the middle of the cache. Technically #embed
tokens are not completely new since they have already been read from the
file and there was #embed annotation token in this place, so set
`reinject` flag for them to silence assertion.

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Differential Revision: https://phabricator.intern.facebook.com/D60251200

Author: Fznamznon

clang/lib/Parse/ParseExpr.cpp

@@ -3597,7 +3597,7 @@ void Parser::injectEmbedTokens() {
     I += 2;
   }
   PP.EnterTokenStream(std::move(Toks), /*DisableMacroExpansion=*/true,
-                      /*IsReinject=*/false);
+                      /*IsReinject=*/true);
   ConsumeAnyToken(/*ConsumeCodeCompletionTok=*/true);
 }
 

clang/test/Preprocessor/embed_weird.cpp

@@ -116,6 +116,14 @@ void f1() {
 }
 #endif
 
+static_assert(_Generic(
+#embed __FILE__ limit(1)
+  , int : 1, default : 0));
+
+static_assert(alignof(typeof(
+#embed __FILE__ limit(1)
+)) == alignof(int));
+
 struct HasChar {
   signed char ch;
 };