feat(3p): Add rapidcheck

Pulled from the commit at current master, overridden to build with clang
and enable gtest and gmock integration.

Change-Id: I10008e8c591bd0c7cc26566b2a050ef2a55bb346
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1282
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

ci-builds.nix

@@ -64,6 +64,7 @@ in lib.fix (self: {
     grpc
     nix
     openldap
+    rapidcheck
   ];
 
   various = with depot; [

third_party/rapidcheck/default.nix

@@ -0,0 +1,21 @@
+{ pkgs, ... }:
+
+(pkgs.callPackage "${pkgs.nixpkgsSrc}/pkgs/development/libraries/rapidcheck" {
+  stdenv = pkgs.llvmPackages.libcxxStdenv;
+}).overrideAttrs (attrs: rec {
+  # follows the versioning scheme of nixpkgs, since rapidcheck does not
+  # provide versioned releases
+  version = "unstable-2020-05-04";
+
+  src = pkgs.fetchFromGitHub {
+    owner = "emil-e";
+    repo = "rapidcheck";
+    rev = "7bc7d302191a4f3d0bf005692677126136e02f60";
+    sha256 = "0khawy2n007yk97ls2qqpna4ly09v6rb6hw72nm16kzk3zbyzh17";
+  };
+
+  cmakeFlags = [
+    "-DRC_ENABLE_GTEST=ON"
+    "-DRC_ENABLE_GMOCK=ON"
+  ];
+})