Put all XSAN bots on the same DEPS-sourced build of Clang.

This adds LLVM, Clang, and compiler-rt (*san tools) to DEPS,
then uses them from xsan_build, building them if needed.

This is similar to how the CMake bots bootstrap CMake if needed.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1693733003

Review URL: https://codereview.chromium.org/1693733003
diff --git a/DEPS b/DEPS
index de69d8c..531f01f 100644
--- a/DEPS
+++ b/DEPS
@@ -50,6 +50,11 @@
 
   # Mojo SDK for Mojo RPC experiments.
   "third_party/externals/mojo/public" : "https://chromium.googlesource.com/external/github.com/domokit/mojo_sdk@172be18ea745ab29aea132e6cb952e726dc32543",
+
+
+  "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",
 }
 
 recursedeps = [ "common" ]