This patch is not intended to introduce any functional change. drvPaths
should remain unchanged; if they do change, it's a bug.

Prior to this patch, a set of packages gets passed through from the
llvmPackages top level function to the individual packages via
callPackages, which is a newScope constructed with some specific
arguments.

As it stands this makes it harder to override dependencies of LLVM; for
example take ncurses. If you want to override it, it is an argument to
libllvm, however, if you override libllvm you then have to write a lot
of code to have correctly overridden clang, given how llvmPackages is
previously composed (out of tools and libraries).

Instead, I propose to make sure that all the dependencies of all
llvmPackages are listed as an inputs to the top leve llvmPackages,
and then the resulting newScope will contain all of them. This in
turn will make `llvmPackages.override` work as expected for any
input to each of the llvm packages.

We'll achieve this by first simplifying the code a bit and ensuring that
all arguments to llvmPackage get forwarded to all packages (via
`{}@args`).

This represents a chance to simplify things a bit so I propose doing it
in two steps:

1. This patch: Simplify argument pass through.
2. (Later): Ensure all arguments to each llvm package are listed in the
   top level `llvm/X/default.nix`.

Once the second patch lands, this means that `(llvmPackages.override {
ncurses = myncurses; }).clang` would consist of a clang whose libllvm
had the ncurses overridden. This is not the case prior to this patch.

Signed-off-by: Peter Waller <[email protected]>
(cherry picked from commit 8164fd6)