[flow] add optional fallback to ExplicitlyIgnored variant

Summary:
This stack will add the capability to provide a backup flowconfig path to use for explicitly ignored files, which will allow us to give IDE support to directory structures like:

```
multiplat/
├── code
│   ├── a.js
│   └── a.native.js
├── .flowconfig // ignores a.native.js
└── other-code
    └──.flowconfig // Checks a.native.js
```

Right now, if we open code/a.native.js we provide no IDE support because it is explicitly ignored by `multiplat/.flowconfig`. This stack will allow us to specify that `a.native.js` is checked by `other-code/.flowconfig` by calling `flow ls --explain`.

NOTE: This is a TEMPORARY solution intended to be a stopgap and will thus not be documented. The real solution here is to unify the flowconfigs and support multiplatform projects properly.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D69859084

fbshipit-source-id: 97df0e2fccaf5d44cfdbefba761d3e123281c8c1

src/commands/lsCommand.ml

@@ -52,7 +52,7 @@ type file_result =
   | ImplicitlyIncluded
   | ExplicitlyIncluded
   | ImplicitlyIgnored
-  | ExplicitlyIgnored
+  | ExplicitlyIgnored of string option
   | ImplicitLib
   | ExplicitLib
   | ConfigFile
@@ -61,7 +61,7 @@ let string_of_file_result = function
   | ImplicitlyIncluded -> "ImplicitlyIncluded"
   | ExplicitlyIncluded -> "ExplicitlyIncluded"
   | ImplicitlyIgnored -> "ImplicitlyIgnored"
-  | ExplicitlyIgnored -> "ExplicitlyIgnored"
+  | ExplicitlyIgnored _ -> "ExplicitlyIgnored"
   | ImplicitLib -> "ImplicitLib"
   | ExplicitLib -> "ExplicitLib"
   | ConfigFile -> "ConfigFile"
@@ -70,7 +70,7 @@ let string_of_file_result_with_padding = function
   | ImplicitlyIncluded -> "ImplicitlyIncluded"
   | ExplicitlyIncluded -> "ExplicitlyIncluded"
   | ImplicitlyIgnored -> "ImplicitlyIgnored "
-  | ExplicitlyIgnored -> "ExplicitlyIgnored "
+  | ExplicitlyIgnored _ -> "ExplicitlyIgnored "
   | ImplicitLib -> "ImplicitLib       "
   | ExplicitLib -> "ExplicitLib       "
   | ConfigFile -> "ConfigFile        "
@@ -89,7 +89,7 @@ let explain ~flowconfig_name ~root ~options ~libs raw_file =
     else if Server_files_js.config_file flowconfig_name root = file then
       ConfigFile
     else if Files.is_ignored options file then
-      ExplicitlyIgnored
+      ExplicitlyIgnored None
     else if String.starts_with ~prefix:root_str file then
       ImplicitlyIncluded
     else if Files.is_included options file then