... consistently.

Further to #307211, allow overriding arguments through llvmPackages.override.

This makes it possible to override any dependency of LLVM or
clang by overriding it on `llvmPackages.override { <dependency> = ...; }`.

This is useful in development or customization where sometimes it is
desirable to turn features on or off.

Without this patch the only way to for example override ncurses was to
do `overriddenLLVM = llvmPackages.llvm.override { ncurses }`, but then
you would have to thread `overriddenLLVM` as dependencies into clang and
other packages, which results in quite a difficult expression to write
correctly in cross compilation scenarios.

Signed-off-by: Peter Waller <[email protected]>
(cherry picked from commit 40a7f21)