Update README and enable C++20 for UnitLICM and UnitLoopInfo

Dependencies

• [2] HVO7VLDM Add coollang
• [3] NJLFHI76 Add cool lang
• [4] 7OVOGA6Q Import LLVM @ 15.0.7
• [5] PR5SVBTF Add readme and some regalloc updates

Change contents

• replacement in coollang/BUILD.bazel at line 26
  [2.282089]→[2.282089:282141](∅→∅)
  − copts = ["-Os", "-std=c++17", "-Wno-register"],
  [2.282089]

  [2.282141]
  + copts = ["-Os", "-Wno-register"],
• edit in coollang/BUILD.bazel at line 67
  [2.282896]→[2.282896:282923](∅→∅)
  − copts = ["-std=c++17"]
• replacement in README.md at line 1
  [4.775]→[4.776:912](∅→∅)
  − This is a monorepo of my compiler projects that use LLVM. It is primarily intended to be built with Bazel, but CMake is also supported.
  [4.775]

  [4.912]
  + This is a monorepo of my compiler projects that use LLVM. It is primarily intended to be built with Bazel, but CMake is also supported on an best-effort basis. This code is compiled from various projects, and certain things are known to not work (tests have not yet been ported, some LLVM passes are not registered yet, etc.)
• edit in README.md at line 11
  [4.1502]→[4.1502:1662](∅→∅)
  − - `llvm/lib/Transforms/Scalar/UnitSCCP.cpp` is a sparse conditional-constant propagation pass

  −

  − Corresponding headers and tests can be found in the usual places
• replacement in MODULE.bazel.lock at line 3
  [4.874041232]→[4.874041232:874041320](∅→∅)
  − "moduleFileHash": "132424f46ca82b16ca90070b250e6ce46fb9e00427df813020b6406b864b73d6",
  [4.874041232]

  [4.874041320]
  + "moduleFileHash": "1e826b3b55357470f24c333578d23b951434a32e25c7b5eb6bc36f54f442a8f5",
• replacement in MODULE.bazel.lock at line 20
  [4.874041767]→[4.874041767:874041797](∅→∅)
  − "name": "llvm-project",
  [4.874041767]

  [4.874041797]
  + "name": "ibn222-llvm-projects",
• replacement in MODULE.bazel.lock at line 23
  [4.874041847]→[4.874041847:874041881](∅→∅)
  − "repoName": "llvm-project",
  [4.874041847]

  [4.874041881]
  + "repoName": "ibn222-llvm-projects",
• replacement in .bazelrc at line 101
  [4.874195547]→[3.85688:85739](∅→∅)
  − build --cxxopt=-std=c++17 --host_cxxopt=-std=c++17
  [4.874195547]

  [4.874195931]
  + build --cxxopt=-std=c++20 --host_cxxopt=-std=c++20