[NFC] Sema: Moved predicate to cpp.

In preparation to make it more involved.

Author: nate-chandler

include/swift/AST/Decl.h

@@ -6029,15 +6029,14 @@ class AbstractStorageDecl : public ValueDecl {
     return getOpaqueReadOwnership() != OpaqueReadOwnership::Borrowed;
   }
 
-  /// Does this storage require a 'read' accessor in its opaque-accessors set?
-  bool requiresOpaqueReadCoroutine() const {
-    return getOpaqueReadOwnership() != OpaqueReadOwnership::Owned;
-  }
+  /// Does this storage require a '_read' accessor in its opaque-accessors set?
+  bool requiresOpaqueReadCoroutine() const;
 
   /// Does this storage require a 'set' accessor in its opaque-accessors set?
   bool requiresOpaqueSetter() const { return supportsMutation(); }
 
-  /// Does this storage require a 'modify' accessor in its opaque-accessors set?
+  /// Does this storage require a '_modify' accessor in its opaque-accessors
+  /// set?
   bool requiresOpaqueModifyCoroutine() const;
 
   /// Does this storage have any explicit observers (willSet or didSet) attached

lib/AST/Decl.cpp

@@ -3006,6 +3006,10 @@ bool AbstractStorageDecl::requiresOpaqueAccessor(AccessorKind kind) const {
   llvm_unreachable("bad accessor kind");
 }
 
+bool AbstractStorageDecl::requiresOpaqueReadCoroutine() const {
+  return getOpaqueReadOwnership() != OpaqueReadOwnership::Owned;
+}
+
 bool AbstractStorageDecl::requiresOpaqueModifyCoroutine() const {
   ASTContext &ctx = getASTContext();
   return evaluateOrDefault(ctx.evaluator,