Compilation Database pulls include from wrong directory.

I'm building Chromium, which is bundled with its own copy of libc++, passed to clang on the command line: -isystem../../buildtools/third_party/libc++/trunk/include -isystem../../buildtools/third_party/libc++abi/trunk/include

Naturally, the build also passes -nostdinc++ to prevent the system libstdc++ from getting pulled in. However, it doesn't appear that the Compilation Database handles this properly. As a result, when the libc++ stdlib.h does `#include_next <stdlib.h>`, it gets the libstdc++ stdlib.h, not the C version as it should, leading to a "Warning: The code model could not parse an included file…" message.

There are a couple of other oddities here:

First, the command-line also specifies a sysroot, but the libstdc++ header that's erroneously pulled in appears to be from /usr/include/c++/6, not from the sysroot. If I leave omit -nostdinc++ on the command line, I get the same error, but instead pointing to libstdc++ in the sysroot like I'd expect, not /usr/include like in Qt Creator.

Second, while the error's the same, the include trace is a little different.

On the command line:

In file included from ../../buildtools/third_party/libc++/trunk/include/memory:653:
In file included from ../../buildtools/third_party/libc++/trunk/include/typeinfo:60:
In file included from ../../buildtools/third_party/libc++/trunk/include/exception:81:
In file included from ../../buildtools/third_party/libc++/trunk/include/cstdlib:85:
In file included from ../../buildtools/third_party/libc++/trunk/include/stdlib.h:97:

In Qt Creator:

In file included from ../../buildtools/third_party/libc++/trunk/include/memory:653:
In file included from ../../buildtools/third_party/libc++/trunk/include/typeinfo:60:
In file included from ../../buildtools/third_party/libc++/trunk/include/exception:81:
In file included from ../../buildtools/third_party/libc++/trunk/include/cstdlib:85:
In file included from ../../buildtools/third_party/libc++/trunk/include/stdlib.h:100:
In file included from ../../buildtools/third_party/libc++/trunk/include/math.h:301:
In file included from ../../buildtools/third_party/libc++/trunk/include/stdlib.h:20:

Happy to collect whatever additional information might be helpful to see what's going on. Is there an easy way to see what command-line is being passed to clang internally?