Fix MSVC build by moving AdapterImpl<void> defs to async.c++

c++/src/kj/async.c++

@@ -138,6 +138,20 @@ Canceler::AdapterBase::~AdapterBase() noexcept(false) {
   }
 }
 
+Canceler::AdapterImpl<void>::AdapterImpl(kj::PromiseFulfiller<void>& fulfiller,
+            Canceler& canceler, kj::Promise<void> inner)
+    : AdapterBase(canceler),
+      fulfiller(fulfiller),
+      inner(inner.then(
+          [&fulfiller]() { fulfiller.fulfill(); },
+          [&fulfiller](kj::Exception&& e) { fulfiller.reject(kj::mv(e)); })
+          .eagerlyEvaluate(nullptr)) {}
+
+void Canceler::AdapterImpl<void>::cancel(kj::Exception&& e) {
+  fulfiller.reject(kj::mv(e));
+  inner = nullptr;
+}
+
 // =======================================================================================
 
 TaskSet::TaskSet(TaskSet::ErrorHandler& errorHandler)

c++/src/kj/async.h

@@ -593,18 +593,10 @@ template <>
 class Canceler::AdapterImpl<void>: public AdapterBase {
 public:
   AdapterImpl(kj::PromiseFulfiller<void>& fulfiller,
-              Canceler& canceler, kj::Promise<void> inner)
-      : AdapterBase(canceler),
-        fulfiller(fulfiller),
-        inner(inner.then(
-            [&fulfiller]() { fulfiller.fulfill(); },
-            [&fulfiller](kj::Exception&& e) { fulfiller.reject(kj::mv(e)); })
-            .eagerlyEvaluate(nullptr)) {}
-
-  void cancel(kj::Exception&& e) override {
-    fulfiller.reject(kj::mv(e));
-    inner = nullptr;
-  }
+              Canceler& canceler, kj::Promise<void> inner);
+  void cancel(kj::Exception&& e) override;
+  // These must be defined in async.c++ to prevent translation units compiled by MSVC from trying to
+  // link with symbols defined in async.c++ merely because they included async.h.
 
 private:
   kj::PromiseFulfiller<void>& fulfiller;