llvmPackages: Make targetLlvmLibraries overridable

[pwaller]

Pull #320261 introduced the possibility to consistently override
dependencies within an llvm package set. This is useful for development
and testing exotic configurations.

Go one step further and enable overriding targetLlvmLibraries.

This makes it possible to write an overlay such as:

overlays = [
  (self: super: {
    llvmPackages = super.llvmPackages.override (prev: {
      targetLlvmLibraries = super.targetPackages.llvmPackages.libraries // {
        compiler-rt = super.targetPackages.llvmPackages.libraries.compiler-rt.override {
          ...
        }
      };
    });
  })
];

... where the overridden compiler-rt will be used in a pkgsLLVM build.

As a straw man, I've done the minimally invasive thing to the code
structure: targetLlvmLibraries is not an explicitly named parameter
for llvmPackages; but it is available in packageSetArgs if passed.
This makes it slightly less discoverable, but this seems like a
reasonable tradeoff considered that modifying this would be a fairly
advanced/esoteric thing to need to do.

In some ways it would be better to have as an explicit parameter with a
default, but the obvious thing won't work because the default needs to
be a non-trivial expression. Potentially we could instead have it as a
defaulted parameter with the value of 'null', and if it's null, then
compute the current thing.

Signed-off-by: Peter Waller [email protected]

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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