Switch from assert to CHECK (#975)

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/parser/parse_tree_fuzzer.cpp

@@ -6,6 +6,7 @@
 #include <cstdint>
 #include <cstring>
 
+#include "common/check.h"
 #include "llvm/ADT/StringRef.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/diagnostics/null_diagnostics.h"
@@ -55,7 +56,7 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
 
   // In the absence of parse errors, we should have exactly as many nodes as
   // tokens.
-  assert(tree.Size() == tokens.Size() && "Unexpected number of tree nodes!");
+  CHECK(tree.Size() == tokens.Size()) << "Unexpected number of tree nodes!";
 
   return 0;
 }