Consolidate token-related range handling to one struct (#5399)

This consolidates Lex::TokenizedBuffer::DumpSemIRRange and
Parse::TreeAndSubtrees::TokenRange into a single InclusiveTokenRange,
also making the OverlapsWithDumpSemIRRange function take the new struct.

I considered switching to `llvm::iterator_range<Lex::TokenIterator>`,
but we often want to see if the range is size one. Using `TokenIterator`
just looked like it'd add a bunch of offsetting to make it work; I view
that as low-value overhead.

For example:

```
  Lex::InclusiveTokenRange token_range = GetSubtreeTokenRange(node_id);
  auto begin_loc = tree_->tokens().TokenToDiagnosticLoc(token_range.begin);
  if (token_range.begin == token_range.end) {
    return begin_loc;
  }
  auto end_loc = tree_->tokens().TokenToDiagnosticLoc(token_range.end);
```

would become:

```
  llvm::iterator_range<Lex::TokenIterator> token_range = GetSubtreeTokenRange(node_id);
  auto begin_loc = tree_->tokens().TokenToDiagnosticLoc(*token_range.begin());
  if (token_range.begin() + 1 == token_range.end()) {
    return begin_loc;
  }
  auto end_loc = tree_->tokens().TokenToDiagnosticLoc(*(token_range.end() - 1));
```

So I'm keeping the bespoke struct.

toolchain/lex/tokenized_buffer.cpp

@@ -440,18 +440,15 @@ auto TokenizedBuffer::TokenToDiagnosticLoc(TokenIndex token) const
   return converted;
 }
 
-auto TokenizedBuffer::OverlapsWithDumpSemIRRange(TokenIndex begin,
-                                                 TokenIndex inclusive_end) const
-    -> bool {
-  if (dump_sem_ir_ranges_.empty()) {
-    return true;
-  }
+auto TokenizedBuffer::OverlapsWithDumpSemIRRange(
+    Lex::InclusiveTokenRange range) const -> bool {
+  CARBON_CHECK(!dump_sem_ir_ranges_.empty());
 
   // Ranges are ordered, so we can decide overlap as soon as we find a range
   // that ends after `begin`.
-  for (auto range : dump_sem_ir_ranges_) {
-    if (range.end > begin) {
-      return range.begin <= inclusive_end;
+  for (auto dump_range : dump_sem_ir_ranges_) {
+    if (dump_range.end >= range.begin) {
+      return dump_range.begin <= range.end;
     }
   }
   return false;