Fix stddef.h and related include errors in runtime

[Artalus]

Right now when started not from system path like /bin or /usr/bin, the tool argues that it cannot locate include files stddef.h, stdint.h etc. when parsing system headers like <iostream>. This of course causes mayhem and incorrect behavior.

The issue is related to clang builtin includes. To quote:

the default location to look for builtin headers is in a path $(dirname /path/to/tool)/../lib/clang/3.3/include relative to the tool binary. This works out-of-the-box for tools running from llvm’s toplevel binary directory after building clang-headers, or if the tool is running from the binary directory of a clang install next to the clang binary.

Another clue from pro.cxx

Berkus Decker, [15.03.18 11:09]
it is not /usr/bin, it is clang-install-dir wherever you've installed it, it is considered as root, since that's the way relocatable toolchain is done in LLVM

Another clue.

Once fixed, apply the same to leak-marker.