[lldb] [lit] Fix finding lld-link when it is not in 'compiler dir'

Fix the build helper to find lld-link via PATH lookup, rather than
making a fragile assumption that it will be present in the 'compiler
directory'.  This fixes tests on Gentoo where clang and lld
are installed in different directories.

Differential Revision: https://reviews.llvm.org/D58001

llvm-svn: 353701