cc-wrapper: -nostdlib does not imply -nostdinc++

[ttuegel]

Motivation for this change

The check for including the C++ standard library headers was nested inside the
check for linking with the C++ standard library. As a result, the -nostdlib
flag incorrectly implied -nostdinc++, which made it virtually impossible to
partially link C++ objects.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[thefloweringash]

I think this broke darwin.network_cmds by including a c++ library with a module map when compiling a c program. This package is doing something weird by enabling modules with -fmodules and setting the language via -x c. I don't know if this is a sensible configuration, but it compiles without error on system clang.

Short reproduction (observed on x86_64-linux and x86_64-darwin):

nix run -f . llvmPackages.tools.libcxxClang -c clang -fmodules -x c -c <(echo "#include <string.h>")