Split out context targets for check and parse. (#3558)

Per request on #3556.

Note, I'm assuming handlers should remain in the same target as the
caller for longer-term performance reasons.

toolchain/parse/parse.cpp

@@ -10,6 +10,10 @@
 
 namespace Carbon::Parse {
 
+// Declare handlers for each parse state.
+#define CARBON_PARSE_STATE(Name) auto Handle##Name(Context& context) -> void;
+#include "toolchain/parse/state.def"
+
 auto HandleInvalid(Context& context) -> void {
   CARBON_FATAL() << "The Invalid state shouldn't be on the stack: "
                  << context.PopState();