Have the AutoRegen.sh script prompt the user for the LLVM src and obj

Have the AutoRegen.sh script prompt the user for the LLVM src and obj
directories if it can't find them. Then, replace those values into the
configure.ac script and pass them to the LLVM_CONFIG_PROJECT so that the
values become the default for llvm_src and llvm_obj variables. In this way
the user is required to input this exactly once, and the scripts take it
from there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27798 91177308-0d34-0410-b5e6-96231b3b80d8