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]]`

testing/base/source_gen_test.cpp

@@ -70,7 +70,7 @@ TEST(SourceGenTest, Identifiers) {
   // Check that repeated calls are different in interesting ways, but have the
   // exact same total bytes.
   ssize_t idents_size_sum = SumSizes(idents);
-  for ([[maybe_unused]] int _ : llvm::seq(10)) {
+  for ([[maybe_unused]] auto _ : llvm::seq(10)) {
     auto idents2 = gen.GetShuffledIdentifiers(1000);
     EXPECT_THAT(idents2, SizeIs(1000));
     // Should be (at least) a different shuffle of identifiers.