[clang][CMake] Add a warning about the Standalone build being deprecated

[MaskRay]

Summary

For me, cmake -Hclang -B/path/to/build currently fails with

CMake Error at CMakeLists.txt:122 (include):
  include could not find requested file:

    GetErrcMessages

-- Found Python3: /usr/bin/python3.9 (found version "3.9.9") found components: Interpreter
CMake Error at /home/maskray/llvm/llvm/utils/llvm-lit/CMakeLists.txt:13 (make_paths_relative):
  Unknown CMake command "make_paths_relative".

Keeping the Standalone build working is now quite a bit of hassle and to
the best of my knowledge not any active contributor typically does. Let's
follow libc++/libc++abi/libunwind (D119341), deprecate and remove the build
mode. We will be able to remove much code related to CLANG_BUILT_STANDALONE.

To keep the disruption minimum, use message(WARNING ... for release/14.x and
message(FATAL_ERROR ... for main.

Similar to D119380 for lld.

[MaskRay]

clang/CMakeLists.txt

16

For the main branch, we can delete this block.

[tstellar]

I would like to keep this supported upstream as this is how we build clang in Fedora. If some people find the stand-alone builds burdensome, then I think it would be best to start a thread on Discourse so we can try to find a solution that works for everyone.

[MaskRay]

In D119383#3309796, @tstellar wrote:

I would like to keep this supported upstream as this is how we build clang in Fedora. If some people find the stand-alone builds burdensome, then I think it would be best to start a thread on Discourse so we can try to find a solution that works for everyone.

I created https://discourse.llvm.org/t/standalone-build-for-clang-lld-fix-or-remove/59977

clang/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 # If we are not building as a part of LLVM, build Clang as an
 # standalone project, using LLVM as an external library:
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(Clang)
   set(CLANG_BUILT_STANDALONE TRUE)
+  message(WARNING "The Standalone build is deprecated in this release. Please
+    use -DLLVM_ENABLE_PROJECTS='...;clang;...' to build clang.")
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(CLANG_BUILT_STANDALONE)
   set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
   set(CMAKE_CXX_EXTENSIONS NO)
 
   # Rely on llvm-config.
   set(LLVM_CONFIG_OUTPUT)
   if(LLVM_CONFIG)
     set (LLVM_CONFIG_FOUND 1)
     message(STATUS "Found LLVM_CONFIG as ${LLVM_CONFIG}")