source dir for LLVM to be specified explicitly. This removes the dependency
on the llvm-config script. If the option is not given, then the scripts use
llvm-config which should be both built and in the PATH. This arrangement
provides a useful default for most developers but also allows the nightly
tester to execute countloc.sh before llvm-config is built and without
altering the PATH.

llvm-svn: 29663