clang 9 broken on macOS

[Kha]

Describe the bug
Compiling any C++ program with the new LLVM 9 package fails on macOS:

    /nix/store/x4z2imh49qa4s8ha8q5jmi2y1yqwk785-clang-wrapper-9.0.0/bin/clang++    -arch x86_64   -o CMakeFiles/cmTC_fc86f.dir/testCXXCompiler.cxx.o -c /Users/vsts/agent/2.155.1/work/1/s/build/CMakeFiles/CMakeTmp/testCXXCompiler.cxx
    dyld: Library not loaded: /nix/store/l5v0m6cdgbga34p5r10wclqssi4nsg1l-clang-9.0.0-lib/lib/libclang-cpp.dylib
      Referenced from: /nix/store/fppa97556f5849wm59jqa7jc7rbvj7sh-clang-9.0.0/bin/clang++
      Reason: image not found

(see also https://dev.azure.com/leanprover/lean4/_build/results?buildId=288)

To Reproduce
(I don't have a Mac available to test a more minimal example)

Additional context
using https://github.com/nixos/nixpkgs/archive/3f4144c30a6351dd79b177328ec4dea03e2ce45f.tar.gz

I believe the issue is the use of . in the following patterns, which doesn't match the seemingly new library libclang-cpp:

nixpkgs/pkgs/development/compilers/llvm/9/clang/default.nix

Lines 73 to 75 in 5ed9d8b

	moveToOutput "lib/libclang.*" "$lib"
	substituteInPlace $out/lib/cmake/clang/ClangTargets-release.cmake \
	--replace "\''${_IMPORT_PREFIX}/lib/libclang." "$lib/lib/libclang."

But again, I don't have a Mac available to test that unfortunately.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: clang_9
# a list of nixos modules affected by the problem
module: