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/tokenized_buffer_fuzzer.cpp

@@ -0,0 +1,56 @@
+// 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/tokenized_buffer.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) {
+  // Ignore large inputs.
+  // TODO: Investigate replacement with an error limit. Content with errors on
+  // escaped quotes (`\"` repeated) have O(M * N) behavior for M errors in a
+  // file length N, so either that will need to also be fixed or M will need to
+  // shrink for large (1MB+) inputs.
+  // This also affects parse_tree_fuzzer.cpp.
+  if (size > 100000) {
+    return 0;
+  }
+  static constexpr llvm::StringLiteral TestFileName = "test.carbon";
+  llvm::vfs::InMemoryFileSystem fs;
+  llvm::StringRef data_ref(reinterpret_cast<const char*>(data), size);
+  CARBON_CHECK(fs.addFile(
+      TestFileName, /*ModificationTime=*/0,
+      llvm::MemoryBuffer::getMemBuffer(data_ref, /*BufferName=*/TestFileName,
+                                       /*RequiresNullTerminator=*/false)));
+  auto source = SourceBuffer::CreateFromFile(fs, TestFileName);
+
+  auto buffer = Lex::TokenizedBuffer::Lex(*source, NullDiagnosticConsumer());
+  if (buffer.has_errors()) {
+    return 0;
+  }
+
+  // Walk the lexed and tokenized buffer to ensure it isn't corrupt in some way.
+  //
+  // TODO: We should enhance this to do more sanity checks on the resulting
+  // token stream.
+  for (Lex::Token token : buffer.tokens()) {
+    int line_number = buffer.GetLineNumber(token);
+    CARBON_CHECK(line_number > 0) << "Invalid line number!";
+    CARBON_CHECK(line_number < INT_MAX) << "Invalid line number!";
+    int column_number = buffer.GetColumnNumber(token);
+    CARBON_CHECK(column_number > 0) << "Invalid line number!";
+    CARBON_CHECK(column_number < INT_MAX) << "Invalid line number!";
+  }
+
+  return 0;
+}
+
+}  // namespace Carbon::Testing