[CMake] Move standalone check so we don't have to reconfigure LLDB

By moving the standalone check into the main CMake file, the whole file
is ignored in a regular (non-standalone) build. This means that you can
make changes to LLDBStandalone.cmake without having to reconfigure a
build in a different directory. This matters when you share one source
repository with different build directories (e.g. release-assert, debug,
standalone).

Differential revision: https://reviews.llvm.org/D64824

llvm-svn: 366346