Support explicit template instantiations for AssignPatch.

Summary:
This diff builds on top of D85391082 which got `gen_patch_ConfigScopes.thrift.cpp` from **128s** -> **78s** in `opt` mode and **86s** -> **61s** in `dev` mode.

While the `extern template` declarations added in D85391082 avoided the nested template instantiations for non-`AssignPatch` cases, it did not help with the `AssignPatch` cases. `gen_patch_ConfigScopes.thrift.cpp` remained still quite expensive, exactly because one of the 3 fields, contains `ClientConfigWrapper.ClientConfigPatchWrapper` which is an `AssignPatch`.

This diff takes the following few steps:
1. Moves `AssignPatch` into `detail`, next to `StructPatch`.
2. Declare `AssignPatch`'s `apply` and `merge` out-of-line, in `AssignPatchImpl.h`, in-line with `StructPatchImpl.h`.
3. Update the gen-patch codegen to explicitly declare and define the instantiations for `apply` and `merge`.
 ---
`fbcode//configerator/structs/servicerouter/configs/if:gen_patch_config_scopes-cpp2-types` was the #1 in the sum of `Action Execution Total Time` as well as `Action Potential Improvement Time` for Thrift targets in critical path of user builds, over the last 2 weeks. [Scuba query](https://fburl.com/scuba/buck2_critical_path_actions_user/t43267f1)

The file that causes this is `gen_patch_ConfigScopes.thrift.cpp`, as can be seen [this query](https://fburl.com/scuba/buck2_critical_path_actions_user/dmwzj03u). With this diff, the 3 costly identifiers improve like this:

|**Identifier**|**Before**|**D85391082**| **After** |
| `gen_patch_ConfigScopes.thrift.cpp (pic) (optimized)` | 128s | 78s | 16s |
| `gen_patch_ConfigScopes.thrift.cpp` | 86s | 61s | 14.5s |
| `gen_patch_ConfigScopes.thrift.cpp (pic)` | 86s | 61s | 15s |

Combined with D85391082, we're looking at ~87.5% improvement in the pic/opt case, which is about ~70% of the time among these 3 files, and ~82.5% improvement in the other 2 cases.

Reviewed By: vitaut

Differential Revision: D85415239

fbshipit-source-id: 0eef1d86ba6ed4bde4a68907f6b5e425a38bbd77

Author: mpark

third-party/thrift/src/thrift/lib/cpp2/op/AssignPatch.h renamed to third-party/thrift/src/thrift/lib/cpp2/op/detail/AssignPatch.h

@@ -74,46 +74,9 @@ class AssignPatch : public BaseAssignPatch<Patch, AssignPatch<Patch>> {
     return *this;
   }
 
-  void apply(T& val) const {
-    if (dynPatch_) {
-      auto value = protocol::asValueStruct<Tag>(val);
-      dynPatch_->apply(value);
-      val = protocol::fromValueStruct<Tag>(value);
-    } else if (auto p = data_.assign()) {
-      val = data_.assign().value();
-    }
-  }
+  void apply(T& val) const;
 
-  void merge(AssignPatch other) {
-    if (dynPatch_ && other.dynPatch_) {
-      XLOG_EVERY_MS(CRITICAL, 10000)
-          << "Merging dynamic patch is not implemented. "
-             "The merged result will be incorrect.\n"
-             "First Patch = "
-          << folly::toPrettyJson(
-                 apache::thrift::protocol::toDynamic(dynPatch_->toObject()))
-          << "\nSecond Patch = "
-          << folly::toPrettyJson(
-                 apache::thrift::protocol::toDynamic(
-                     other.dynPatch_->toObject()));
-
-      // Do nothing, which is the old behavior
-      return;
-    }
-
-    if (!other.dynPatch_) {
-      if (auto p = other.data_.assign()) {
-        assign(std::move(*p));
-      }
-      return;
-    }
-
-    if (auto p = data_.assign()) {
-      other.apply(*p);
-    } else {
-      dynPatch_ = std::move(other.dynPatch_);
-    }
-  }
+  void merge(AssignPatch other);
 
   /// @cond
   template <class Protocol>

third-party/thrift/src/thrift/lib/cpp2/op/detail/AssignPatchImpl.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <thrift/lib/cpp2/op/detail/AssignPatch.h>
+
+namespace apache::thrift::op::detail {
+
+template <typename Patch>
+void AssignPatch<Patch>::apply(T& val) const {
+  if (dynPatch_) {
+    auto value = protocol::asValueStruct<Tag>(val);
+    dynPatch_->apply(value);
+    val = protocol::fromValueStruct<Tag>(value);
+  } else if (auto p = data_.assign()) {
+    val = data_.assign().value();
+  }
+}
+
+template <typename Patch>
+void AssignPatch<Patch>::merge(AssignPatch other) {
+  if (dynPatch_ && other.dynPatch_) {
+    XLOG_EVERY_MS(CRITICAL, 10000)
+        << "Merging dynamic patch is not implemented. "
+           "The merged result will be incorrect.\n"
+           "First Patch = "
+        << folly::toPrettyJson(
+               apache::thrift::protocol::toDynamic(dynPatch_->toObject()))
+        << "\nSecond Patch = "
+        << folly::toPrettyJson(
+               apache::thrift::protocol::toDynamic(
+                   other.dynPatch_->toObject()));
+
+    // Do nothing, which is the old behavior
+    return;
+  }
+
+  if (!other.dynPatch_) {
+    if (auto p = other.data_.assign()) {
+      assign(std::move(*p));
+    }
+    return;
+  }
+
+  if (auto p = data_.assign()) {
+    other.apply(*p);
+  } else {
+    dynPatch_ = std::move(other.dynPatch_);
+  }
+}
+
+} // namespace apache::thrift::op::detail