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/sem_ir/builtin_function_kind.cpp

@@ -91,7 +91,8 @@ struct AnyInt {
 struct AnyFloat {
   static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
       -> bool {
-    if (BuiltinType<InstId::BuiltinFloatType>::Check(sem_ir, state, type_id)) {
+    if (BuiltinType<InstId::BuiltinLegacyFloatType>::Check(sem_ir, state,
+                                                           type_id)) {
       return true;
     }
     return sem_ir.types().Is<FloatType>(type_id);