Rename the builtin FloatType to LegacyFloatType, Error to ErrorInst (#4555)

This is for more clearly distinct names, and to make it a clearer
transition from `BuiltinInst` for name conflicts. `FloatType` is also an
instruction, and we have `Carbon::Error` (common/error.h). This avoids
affecting tests, although the name is embedded in the builtin test.

In `LegacyFloatType`, `Legacy` because I was having trouble coming up
with a more appropriate name. I'm not clear this is a `FloatLiteralType`
at present, it needs some work to mirror `IntLiteralType`.

In `ErrorInst`, the suffix `Inst` was discussed as good and similar to
`BuiltinInst` (although I'm trying to get rid of that).

toolchain/check/scope_stack.cpp

@@ -122,7 +122,7 @@ auto ScopeStack::LookupInLexicalScopes(SemIR::NameId name_id)
 
   // If we have no lexical results, check all non-lexical scopes.
   if (lexical_results.empty()) {
-    return {LexicalLookupHasLoadError() ? SemIR::InstId::BuiltinError
+    return {LexicalLookupHasLoadError() ? SemIR::InstId::BuiltinErrorInst
                                         : SemIR::InstId::Invalid,
             non_lexical_scope_stack_};
   }