Re-land "[lldb/CMake] Change how we deal with optional dependencies"

Recently there has been some discussion about how we deal with optional
dependencies in LLDB. The approach in LLVM is to make things work out of
the box. If the dependency isn't there, we move on silently.

That's not true for LLDB. Unless you explicitly disable the dependency
with LLDB_ENABLE_*, you'll get a configuration-time error. The
historical reason for this is that LLDB's dependencies have a much
broader impact, think about Python for example which is required to run
the test suite.

The current approach can be frustrating from a user experience
perspective. Sometimes you just want to ensure LLDB builds with a change
in clang.

This patch changes the optional dependencies (with the exception of
Python) to a new scheme. The LLDB_ENABLE_* now takes three values: On,
Off or Auto, with the latter being the default. On and Off behave the
same as today, forcing the dependency to be enabled or disabled. If the
dependency is set to On bu...