[CMake] Generate LLDB_REVISION at build time

Summary:
This alters the generation of LLDB_REVISION to be heavily based on how clang generates its version header. There are two benefits of this aproach.

(1) The LLDB_REVISION is generated at build time, so it will be updated after an SCM pull/update even if CMake doesn't re-run
(2) This works on Windows

As noted this code is a simplified implementation of the code from clang.

Reviewers: tfiala, zturner

Subscribers: beanz, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D24846

llvm-svn: 282314