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/handle_array.cpp

@@ -46,7 +46,7 @@ auto HandleParseNode(Context& context, Parse::ArrayExprId node_id) -> bool {
   if (!context.constant_values().Get(bound_inst_id).is_constant()) {
     CARBON_DIAGNOSTIC(InvalidArrayExpr, Error, "array bound is not a constant");
     context.emitter().Emit(bound_inst_id, InvalidArrayExpr);
-    context.node_stack().Push(node_id, SemIR::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
     return true;
   }