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/interface.cpp

@@ -18,7 +18,7 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
     // This should only happen if the interface is erroneously defined more than
     // once.
     // TODO: Find a way to CHECK this.
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // The interface type is the type of `Self`.