Make it possible to run dotest on Linux without any parameters

dotest will select clang-3.5 by default and fall back on clang/gcc
dotest will look for the lldb executable in the typical cmake
output locations:
{lldb}/../../../build/bin (next to llvm directory)
{lldb}/../../../build/host/bin (next to llvm directory)
{lldb}/../build/bin (next to lldb directory)
{lldb}/../build/host/bin (next to lldb directory)

llvm-svn: 237632