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

[Petr Hosek via cfe-commits]

petrhosek wrote:

> 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.

That's exactly what we do in Fuchsia, we build runtimes in [stage 1](https://github.com/llvm/llvm-project/blob/2b9e947d4346ad03328a31f90b9056837c042d1b/clang/cmake/caches/Fuchsia.cmake) and then use them in [stage 2](https://github.com/llvm/llvm-project/blob/2b9e947d4346ad03328a31f90b9056837c042d1b/clang/cmake/caches/Fuchsia-stage2.cmake).

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