build: Downgrade LLVM 17.0.6 -> 16.0.0

settings.bzl

@@ -1,5 +1,5 @@
 # Keep LLVM versions list in sync with setup_llvm.bzl
 ASAN_LINKOPTS = [
-    "-Wl,-rpath,@loader_path/../../../../../../external/llvm_toolchain_llvm/lib/clang/17.0.6/lib/darwin",
-    "-Wl,-rpath,@loader_path/../../../../../../external/llvm_toolchain_llvm/lib/clang/17.0.6/lib/linux",
-]
+    "-Wl,-rpath,@loader_path/../../../../../../external/llvm_toolchain_llvm/lib/clang/16.0.0/lib/darwin",
+    "-Wl,-rpath,@loader_path/../../../../../../external/llvm_toolchain_llvm/lib/clang/16.0.0/lib/linux",
+]

setup_llvm.bzl

@@ -3,11 +3,19 @@ load("@toolchains_llvm//toolchain:rules.bzl", grailbio_llvm_toolchain = "llvm_to
 def setup_llvm_toolchain(name):
     # NOTE: The ASan build uses paths which involve the version.
     # Keep the version list in sync with settings.bzl
+    # At the time of bumping this, the latest version is 17.0.6,
+    # but we use 16.0.0 because:
+    # - Versions later than 16.0.0 are built on Ubuntu 22.04 or newer,
+    #   whereas 16.0.0 is built on Ubuntu 18.04. The newer binaries require
+    #   a newer glibc version which isn't currently present on Buildkite machines.
+    # - Our release pipeline uses Ubuntu 18.04 so that the binaries work on
+    #   older Debian and Ubuntu versions. Using newer toolchain binaries would
+    #   hence not work in our release pipeline.
     mapping = {
-        "linux-aarch64": {"version": "17.0.6", "triple": "aarch64-linux-gnu", "sha256": "6dd62762285326f223f40b8e4f2864b5c372de3f7de0731cb7cd55ca5287b75a"},
-        "linux-x86_64": {"version": "17.0.6", "triple": "x86_64-linux-gnu-ubuntu-22.04", "sha256": "884ee67d647d77e58740c1e645649e29ae9e8a6fe87c1376be0f3a30f3cc9ab3"},
-        "darwin-aarch64": {"version": "17.0.6", "triple": "arm64-apple-darwin22.0", "sha256": "1264eb3c2a4a6d5e9354c3e5dc5cb6c6481e678f6456f36d2e0e566e9400fcad"},
-        "darwin-arm64": {"version": "17.0.6", "triple": "arm64-apple-darwin22.0", "sha256": "1264eb3c2a4a6d5e9354c3e5dc5cb6c6481e678f6456f36d2e0e566e9400fcad"},
+        "linux-aarch64": {"version": "16.0.0", "triple": "aarch64-linux-gnu", "sha256": "b750ba3120e6153fc5b316092f19b52cf3eb64e19e5f44bd1b962cb54a20cf0a"},
+        "linux-x86_64": {"version": "16.0.0", "triple": "x86_64-linux-gnu-ubuntu-18.04", "sha256": "2b8a69798e8dddeb57a186ecac217a35ea45607cb2b3cf30014431cff4340ad1"},
+        "darwin-aarch64": {"version": "16.0.0", "triple": "arm64-apple-darwin22.0", "sha256": "2041587b90626a4a87f0de14a5842c14c6c3374f42c8ed12726ef017416409d9"},
+        "darwin-arm64": {"version": "16.0.0", "triple": "arm64-apple-darwin22.0", "sha256": "2041587b90626a4a87f0de14a5842c14c6c3374f42c8ed12726ef017416409d9"},
     }
     llvm_versions, sha256, strip_prefix, urls = {}, {}, {}, {}
     for (k, v) in mapping.items():