[clang] [ToolChains/NetBSD] Support relative libc++ header path

Support locating the libc++ header files relatively to the clang
executable, in addition to the default system path.  This is meant
to cover two use cases: running just-built clang from the install
directory, and running installed clang from non-standard location
(e.g. /usr/local).

This is the first step towards ensuring that tests of more LLVM projects
can work out-of-the-box within the build tree, and use the correct set
of headers (rather than e.g. mixing just-built clang+libcxx with system
install of libcxx).  It avoids requiring the user to hack around missing
include paths, or LLVM build system to replicate system-specific C++
library defaults in order to append appropriate paths implicitly.

Differential Revision: https://reviews.llvm.org/D58592

llvm-svn: 355282