[merge] only try to read/create multilink files when --merge is specified

Summary:
Thanks to the logging introduced in D5293334 (or because of, depending on your liking of spam), I noticed that there lots of errors being logged of the form

  Couldn't read multilink file '/home/jul/infer/infer/tests/codetoanalyze/cpp/errors/infer-out/attributes/d0/multilink.txt': /home/jul/infer/infer/tests/codetoanalyze/cpp/errors/infer-out/attributes/d0/multilink.txt: No such file or directory

I don't think it makes sense to care about multilink files except when `--merge` is specified.

Also introduced a .mli and self-documented a function.

Reviewed By: akotulski

Differential Revision: D5310129

fbshipit-source-id: c3a6276

infer/src/backend/InferAnalyze.re

@@ -132,7 +132,7 @@ let main makefile => {
   BuiltinDefn.init ();
   RegisterCheckers.register ();
   switch Config.modified_targets {
-  | Some file => MergeCapture.modified_file file
+  | Some file => MergeCapture.record_modified_targets_from_file file
   | None => ()
   };
   switch Config.cluster_cmdline {

infer/src/backend/mergeCapture.ml

@@ -15,8 +15,6 @@ module F = Format
 
 (** Module to merge the results of capture for different buck targets. *)
 
-let use_multilinks = true
-
 (** Flag to control whether the timestamp of symbolic links
     is used to determine whether a captured directory needs to be merged. *)
 let check_timestamp_of_symlinks = true
@@ -27,7 +25,7 @@ let infer_deps () = Filename.concat Config.results_dir Config.buck_infer_deps_fi
 
 let modified_targets = ref String.Set.empty
 
-let modified_file file =
+let record_modified_targets_from_file file =
   match Utils.read_file file with
   | Ok targets ->
       modified_targets := List.fold ~f:String.Set.add ~init:String.Set.empty targets
@@ -118,7 +116,7 @@ let rec slink ~stats ~skiplevels src dst =
         items
     end
   else if skiplevels > 0 then ()
-  else if use_multilinks && Filename.check_suffix dst ".attr"
+  else if Config.merge && Filename.check_suffix dst ".attr"
   then add_multilink_attr ~stats src dst
   else create_link ~stats src dst
 

infer/src/backend/mergeCapture.mli

@@ -0,0 +1,14 @@
+(*
+ * Copyright (c) 2017 - present Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *)
+
+open! IStd
+
+val record_modified_targets_from_file : string -> unit
+
+val merge_captured_targets : unit -> unit