Rename lexer dir to lex (#3179)

Continuing with #3070. Just a dir and file rename (only prefix change is
lexer_file_test). Everything in the lex dir should be marked as a move.

Note, I think this closes #3070. There may still be further cleanup
later, but the organizational changes suggested there are being
completed.

---------

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

toolchain/lex/string_literal_fuzzer.cpp

@@ -0,0 +1,43 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cstring>
+
+#include "common/check.h"
+#include "llvm/ADT/StringRef.h"
+#include "toolchain/diagnostics/null_diagnostics.h"
+#include "toolchain/lex/string_literal.h"
+
+namespace Carbon::Testing {
+
+// NOLINTNEXTLINE: Match the documented fuzzer entry point declaration style.
+extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
+                                      std::size_t size) {
+  auto token = Lex::StringLiteral::Lex(
+      llvm::StringRef(reinterpret_cast<const char*>(data), size));
+  if (!token) {
+    // Lexically not a string literal.
+    return 0;
+  }
+
+  if (!token->is_terminated()) {
+    // Found errors while parsing.
+    return 0;
+  }
+
+  fprintf(stderr, "valid: %d\n", token->is_terminated());
+  fprintf(stderr, "size: %lu\n", token->text().size());
+  fprintf(stderr, "text: %s\n", token->text().str().c_str());
+
+  // Check multiline flag was computed correctly.
+  CARBON_CHECK(token->is_multi_line() == token->text().contains('\n'));
+
+  volatile auto value =
+      token->ComputeValue(NullDiagnosticEmitter<const char*>());
+  (void)value;
+
+  return 0;
+}
+
+}  // namespace Carbon::Testing