InstrProf: lit.cfg magic for running tests from source

Implement magic in compiler-rt to enable llvm-lit to be invoked on the
source tree in lib/profile.  This relies on a paired commit in the llvm
tree that exposes a new built-in parameter.

<rdar://problem/16458307>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205263 91177308-0d34-0410-b5e6-96231b3b80d8