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/deduce.cpp

@@ -322,7 +322,7 @@ auto DeductionContext::Deduce() -> bool {
       DiagnosticAnnotationScope annotate_diagnostics(&context().emitter(),
                                                      note_initializing_param);
       arg_id = ConvertToValueOfType(context(), loc_id_, arg_id, param_type_id);
-      if (arg_id == SemIR::InstId::BuiltinError) {
+      if (arg_id == SemIR::InstId::BuiltinErrorInst) {
         return false;
       }
     }