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/import_ref.cpp

@@ -993,7 +993,7 @@ class ImportRefResolver {
         break;
       }
       default: {
-        if (const_inst_id == SemIR::InstId::BuiltinError) {
+        if (const_inst_id == SemIR::InstId::BuiltinErrorInst) {
           return SemIR::NameScopeId::Invalid;
         }
         break;