Make llvmdo and llvmgrep invulnerable to where they are run from by getting

the llvm source root from the llvm-config command. The dependency now is
that the correct llvm-config command is in the path. For most developers
this is the case.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29630 91177308-0d34-0410-b5e6-96231b3b80d8