[libc] Fix deprecated operator"" syntax (llvm#123259)

Author: frobtech

libc/test/src/unistd/getopt_test.cpp

@@ -79,7 +79,7 @@ struct LlvmLibcGetoptTest : public LIBC_NAMESPACE::testing::Test {
 
 // This is safe because getopt doesn't currently permute argv like GNU's getopt
 // does so this just helps silence warnings.
-char *operator"" _c(const char *c, size_t) { return const_cast<char *>(c); }
+char *operator""_c(const char *c, size_t) { return const_cast<char *>(c); }
 
 TEST_F(LlvmLibcGetoptTest, NoMatch) {
   array<char *, 3> argv{"prog"_c, "arg1"_c, nullptr};