llvmbot monitor: Add flang libcxx silent bot

This is what is replacing the latest clang bot.

Change-Id: I5d8baa84f045cd44a862c3b7471fc03d569f7c3e
diff --git a/monitor/linaro.json b/monitor/linaro.json
index 8a5cd40..8690659 100644
--- a/monitor/linaro.json
+++ b/monitor/linaro.json
@@ -78,6 +78,20 @@
     ]
   },
   {
+    "name": "LLVM Silent Master",
+    "base_url": "https://lab.llvm.org/staging",
+    "builder_url": "builders",
+    "build_url": "builds",
+    "builders": [
+      {
+        "name": "Flang",
+        "bots": [
+          { "name": "flang-aarch64-libcxx" }
+        ]
+      }
+    ]
+  },
+  {
     "name": "Buildkite",
     "base_url": "https://www.buildkite.com",
     "buildkite_org": "llvm-project",