Add Dump functions to Check, Parse, and Lex (#4669)

- Provide `Check::Dump(context, arg)` and similar.
- gdb and lldb should do contextual lookup, and `call Dump(*this,
Lex::TokenIndex::Invalid)` has been tested with gdb.
- Since this is only for debug, keeps the functions fully separated from
code.
- Uses alwayslink to ensure objects are correctly linked, even though
there are no calls.
- `-Wno-missing-prototypes` is needed when we don't have forward
declarations.
- Code is not linked in opt builds, using `#ifndef NDEBUG`.
- This probably could be doing something in BUILD files with a
`select()`, but the `#ifndef` seemed easier.

This is based on #4620, but uses free functions instead of member
functions.

Co-authored-by: Dana Jansens <danakj@orodu.net>

---------

Co-authored-by: danakj <danakj@orodu.net>

toolchain/parse/context.cpp

@@ -481,7 +481,7 @@ auto Context::PrintForStackDump(llvm::raw_ostream& output) const -> void {
 
 auto Context::PrintTokenForStackDump(llvm::raw_ostream& output,
                                      Lex::TokenIndex token) const -> void {
-  output << " @ " << tokens_->GetLineNumber(tokens_->GetLine(token)) << ":"
+  output << " @ " << tokens_->GetLineNumber(token) << ":"
          << tokens_->GetColumnNumber(token) << ": token " << token << " : "
          << tokens_->GetKind(token) << "\n";
 }