[CMake] Support generating Config.h

Summary:
This patch removes the hand maintained config files in favor of auto-generating the config file. We will still need to maintain the defines for the Xcode builds on Mac, but all CMake builds use the generated header instead.

This will enable finer grained platform support tests and enable supporting LLDB on more platforms with less manual maintenance.

I have only tested this patch on Darwin, and any help testing it out on other platforms would be greatly appreciated. I've probably messed something up somewhere.

Reviewers: labath, zturner

Reviewed By: labath

Subscribers: krytarowski, emaste, srhines, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D31969

llvm-svn: 300372