folly: workaround AsyncTimeoutTest ICE with gcc 8

Summary:
```
folly/io/async/test/AsyncTimeoutTest.cpp: In lambda function:
folly/io/async/test/AsyncTimeoutTest.cpp:31:70: internal compiler error: Segmentation fault
       AsyncTimeout::make(manager, [&]() noexcept { value = expected; });
                                                                      ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://bugzilla.redhat.com/bugzilla> for instructions.
```

I'm using:

```
 $ /opt/rh/devtoolset-8/root/usr/bin/c++ --version
c++ (GCC) 8.2.1 20180801 (Red Hat 8.2.1-2)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```

This has been fixed in gcc 8.3, but the workaround is to explicitly
name the captures as is done in this diff.

Reviewed By: yfeldblum

Differential Revision: D14702587

fbshipit-source-id: 1998ef5de8eb4df520edcc41fdc9cd9547dec824

folly/io/async/test/AsyncTimeoutTest.cpp

@@ -26,7 +26,7 @@ TEST(AsyncTimeout, make) {
   EventBase manager;
 
   auto observer =
-      AsyncTimeout::make(manager, [&]() noexcept { value = expected; });
+      AsyncTimeout::make(manager, [&value]() noexcept { value = expected; });
 
   observer->scheduleTimeout(std::chrono::milliseconds(100));
 
@@ -41,7 +41,7 @@ TEST(AsyncTimeout, schedule) {
   EventBase manager;
 
   auto observer = AsyncTimeout::schedule(
-      std::chrono::milliseconds(100), manager, [&]() noexcept {
+      std::chrono::milliseconds(100), manager, [&value]() noexcept {
         value = expected;
       });
 
@@ -56,7 +56,7 @@ TEST(AsyncTimeout, schedule_immediate) {
   EventBase manager;
 
   auto observer = AsyncTimeout::schedule(
-      std::chrono::milliseconds(0), manager, [&]() noexcept {
+      std::chrono::milliseconds(0), manager, [&value]() noexcept {
         value = expected;
       });
 
@@ -70,7 +70,7 @@ TEST(AsyncTimeout, cancel_make) {
   EventBase manager;
 
   auto observer =
-      AsyncTimeout::make(manager, [&]() noexcept { value = expected; });
+      AsyncTimeout::make(manager, [&value]() noexcept { value = expected; });
 
   std::weak_ptr<RequestContext> rctx_weak_ptr;
 
@@ -102,7 +102,7 @@ TEST(AsyncTimeout, cancel_schedule) {
     rctx_weak_ptr = RequestContext::saveContext();
 
     observer = AsyncTimeout::schedule(
-        std::chrono::milliseconds(100), manager, [&]() noexcept {
+        std::chrono::milliseconds(100), manager, [&value]() noexcept {
           value = expected;
         });