clang_6: fix sanitizers under libstdc++

[Ralith]

Motivation for this change

Fix headers and runtime libs being lost due to #39743. Sanitizer test is now passing under both clang stdenvs.

cc @Ericson2314

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[Ericson2314]

Great job! I'm going to investigate whether all this stuff is a mass rebuild on master where llvm 5 is still used.

[Ralith]

Is this blocked? As of #41081 I'm pretty sure the sanitizers have been outright broken for clang/libstdc++ (the more common configuration) on master, which is a step backwards from the initial state of affairs.

[Ericson2314]

Nah sorry just away from computer over weekend. Rebasing and merging now.

[Ericson2314]

Oh staging was (finally!) merged, so no rebase even needed

[LnL7]

This broke the cc-wrapper test

checking whether compiler uses NIX_CFLAGS_COMPILE... ok
checking whether compiler uses NIX_LDFLAGS... ok
In file included from <built-in>:340:
<command line>:1:9: warning: '_FORTIFY_SOURCE' macro redefined [-Wmacro-redefined]
#define _FORTIFY_SOURCE 2
        ^
<built-in>:328:9: note: previous definition is here
#define _FORTIFY_SOURCE 0
        ^
/nix/store/3c3vrabg4yy87y4qx8zsy5x2vdg132j5-sanitizers.c:1:10: fatal error: 'sanitizer/asan_interface.h' file not found
#include <sanitizer/asan_interface.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning and 1 error generated.
builder for '/nix/store/g5m71w28lj8sri93ad7pd8zyia161azm-cc-wrapper-test.drv' failed with exit code 1

[Ericson2314]

Guess we will need some conditions. @Ralith know what's up?