Invalid comment cleanup (#4836)

This is a followup from #4834, I searched for "invalid" uses in our
codebase. This is mostly changing comments, and a couple debug
functions, but shouldn't affect testable behavior.

Note a couple things I'll highlight as not changing (but could) are:
- `ReturnTypeInfo::is_valid`
- `"invalid"` uses in the formatter
- `AddInvalid` for `!has_value` in `inst_fingerprinter` (because the
cases it's called sound invalid-ish)

toolchain/check/dump.cpp

@@ -31,7 +31,7 @@ namespace Carbon::Check {
 
 static auto DumpNoNewline(const Context& context, SemIR::LocId loc_id) -> void {
   if (!loc_id.has_value()) {
-    llvm::errs() << "LocId(invalid)";
+    llvm::errs() << "LocId(<none>)";
     return;
   }