Cleanup pass over llvm::seq uses (#5185)

I was thinking about this after `seq` changes in #5182, and looked for
other uses that might be replaceable. Here's the resulting cleanup
around `seq`:

- Switch to `enumerate` or `zip` when possible.
- `int _` -> `auto _` (it's typically a `size_t`, but there's no reason
to cast when unused)
- Fix a case of cast style `(size_t)...` -> `static_cast<size_t>(...)`
- Switch `(void)close_children_count` to `[[maybe_unused]]`

toolchain/lex/tokenized_buffer_test.cpp

@@ -1034,7 +1034,7 @@ TEST_F(LexerTest, TypeLiteralTooManyDigits) {
   // A 128-bit APInt should be plenty large, but if needed in the future it can
   // be widened without issue.
   llvm::APInt bits = llvm::APInt::getZero(128);
-  for ([[maybe_unused]] int _ : llvm::seq(1, 30)) {
+  for ([[maybe_unused]] auto _ : llvm::seq(1, 30)) {
     code.append("9");
     bits = bits * 10 + 9;
     auto [buffer, value_stores] =
@@ -1160,7 +1160,7 @@ TEST_F(LexerTest, DiagnosticFileTooLarge) {
   static constexpr size_t NumLines = 10'000'000;
   std::string input;
   input.reserve(NumLines * 3);
-  for ([[maybe_unused]] int _ : llvm::seq(NumLines)) {
+  for ([[maybe_unused]] auto _ : llvm::seq(NumLines)) {
     input += "{}\n";
   }
   EXPECT_CALL(consumer,