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/decl_name_stack.cpp

@@ -19,7 +19,7 @@ namespace Carbon::Check {
 auto DeclNameStack::NameContext::prev_inst_id() -> SemIR::InstId {
   switch (state) {
     case NameContext::State::Error:
-      // The name is invalid and a diagnostic has already been emitted.
+      // The name is malformed and a diagnostic has already been emitted.
       return SemIR::InstId::None;
 
     case NameContext::State::Empty: