Move control flow block functions to their own file. (#4921)

context.cpp is getting large, so I'm looking at a few ways to cut out
clusters of functions. This felt like a logical cluster of functions to
move to their own file.

Note this doesn't touch the implementation at all, beyond what's needed
to change from `Context` members to context args.

toolchain/check/handle_function.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
+#include "toolchain/check/control_flow.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/decl_introducer_state.h"
 #include "toolchain/check/decl_name_stack.h"
@@ -445,7 +446,7 @@ auto HandleParseNode(Context& context, Parse::FunctionDefinitionId node_id)
 
   // If the `}` of the function is reachable, reject if we need a return value
   // and otherwise add an implicit `return;`.
-  if (context.is_current_position_reachable()) {
+  if (IsCurrentPositionReachable(context)) {
     if (context.functions()
             .Get(function_id)
             .return_slot_pattern_id.has_value()) {