Move llvm from deps to deps_os

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index a723d8d7ce3682b9f43721b257150bcffb88c613..bbbee299d06f9dc8f9e9a2fb692a1d5c32bd2364 100644
--- a/DEPS
+++ b/DEPS
@@ -50,13 +50,17 @@ deps = {
 
   # Mojo SDK for Mojo RPC experiments.
   "third_party/externals/mojo/public" : "https://chromium.googlesource.com/external/github.com/domokit/mojo_sdk@172be18ea745ab29aea132e6cb952e726dc32543",
+}
 
-  # xSAN bots build Clang from scratch.
-  "third_party/externals/llvm": "https://llvm.googlesource.com/llvm@release_38",
-  "third_party/externals/llvm/tools/clang": "https://llvm.googlesource.com/clang@release_38",
-  "third_party/externals/llvm/projects/compiler-rt": "https://llvm.googlesource.com/compiler-rt@release_38",
-  "third_party/externals/llvm/projects/libcxx": "https://llvm.googlesource.com/libcxx@release_38",
-  "third_party/externals/llvm/projects/libcxxabi": "https://llvm.googlesource.com/libcxxabi@release_38",
+deps_os = {
+  'llvm': {
+    # xSAN bots build Clang from scratch.
+    "third_party/externals/llvm": "https://llvm.googlesource.com/llvm@release_38",
+    "third_party/externals/llvm/tools/clang": "https://llvm.googlesource.com/clang@release_38",
+    "third_party/externals/llvm/projects/compiler-rt": "https://llvm.googlesource.com/compiler-rt@release_38",
+    "third_party/externals/llvm/projects/libcxx": "https://llvm.googlesource.com/libcxx@release_38",
+    "third_party/externals/llvm/projects/libcxxabi": "https://llvm.googlesource.com/libcxxabi@release_38",
+  }
 }
 
 recursedeps = [ "common" ]