chromium: control llvmPackages version selection with a string

Sometimes we access `llvmPackages` via `pkgs`, and other times via
`pkgsFooBar`, so unfortunately a string (attrname) is the only way
to have a single point of control over the LLVM version used for
both buildPlatform and hostPlatform.

pkgs/applications/networking/browsers/chromium/common.nix

@@ -1,4 +1,7 @@
 { stdenv, lib, fetchurl, fetchpatch
+, buildPackages
+, pkgsBuildBuild
+, pkgsBuildTarget
 # Channel data:
 , channel, upstream-info
 # Helper functions:
@@ -8,7 +11,7 @@
 , ninja, pkg-config
 , python3, perl
 , which
-, llvmPackages
+, llvmPackages_attrName
 , rustc
 # postPatch:
 , pkgsBuildHost
@@ -130,7 +133,7 @@ let
       ninja pkg-config
       python3WithPackages perl
       which
-      llvmPackages.bintools
+      buildPackages.${llvmPackages_attrName}.bintools
       bison gperf
     ];
 
@@ -308,7 +311,7 @@ let
       rtc_use_pipewire = true;
       # Disable PGO because the profile data requires a newer compiler version (LLVM 14 isn't sufficient):
       chrome_pgo_phase = 0;
-      clang_base_path = "${llvmPackages.stdenv.cc}";
+      clang_base_path = "${pkgsBuildTarget.${llvmPackages_attrName}.stdenv.cc}";
       use_qt = false;
       # To fix the build as we don't provide libffi_pic.a
       # (ld.lld: error: unable to find library -l:libffi_pic.a):

pkgs/applications/networking/browsers/chromium/default.nix

@@ -18,11 +18,15 @@
 , commandLineArgs ? ""
 , pkgsBuildTarget
 , pkgsBuildBuild
+, pkgs
 }:
 
 let
-  llvmPackages = pkgsBuildTarget.llvmPackages_16;
-  stdenv = llvmPackages.stdenv;
+  # Sometimes we access `llvmPackages` via `pkgs`, and other times
+  # via `pkgsFooBar`, so a string (attrname) is the only way to have
+  # a single point of control over the LLVM version used.
+  llvmPackages_attrName = "llvmPackages_16";
+  stdenv = pkgs.${llvmPackages_attrName}.stdenv;
 
   upstream-info = (lib.importJSON ./upstream-info.json).${channel};
 
@@ -44,7 +48,7 @@ let
   callPackage = newScope chromium;
 
   chromium = rec {
-    inherit stdenv llvmPackages upstream-info;
+    inherit stdenv llvmPackages_attrName upstream-info;
 
     mkChromiumDerivation = callPackage ./common.nix ({
       inherit channel chromiumVersionAtLeast versionRange;