[clang] [CMake] Add a cache file for building a highly-optimized LLVM toolchain (PR #117802)

[Shoaib Meenai via cfe-commits]

https://github.com/smeenai commented:

The runtimes build already builds the toolchain and then builds the runtimes with the just-built toolchain. Can we build the LTO-optimized libc++ as part of stage1 and have stage2 be what the current stage3 is, to avoid any redundancy? I really like the direction here otherwise.

https://github.com/llvm/llvm-project/pull/117802