[lldb] Enable cmake policy CMP0077 for option()

In lldb, explicitly set the "option() honors normal variables" CMake policy. This applies for
standalone lldb builds and matches what llvm, clang, etc do. This prevents potentially unwanted
clearing of variables like `LLVM_ENABLE_WARNINGS`, and also prevents unnecessary build warnings.

See: https://cmake.org/cmake/help/latest/policy/CMP0077.html

Differential Revision: https://reviews.llvm.org/D89614