[CMake] Allow version overrides with -DLLDB_VERSION_MAJOR/MINOR/PATCH/SUFFIX

Summary:
This follows the approach in Clang. If no overrides are given, LLDB_VERSION_* is inferred from LLVM_VERSION_*. This mimics the current behaviour (PACKAGE_VERSION itself is generated from LLVM_VERSION_*).
For in-tree builds LLVM_VERSION_* will be defined at this point already. For standalone builds, LLDBConfig.cmake is included after LLDBStandalone.cmake which includes LLVMConfig.cmake.

Reviewers: labath, xiaobai

Subscribers: mgorny, friss, aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D54333

llvm-svn: 346668