Deprecate LLVM_BUILD_EXTERNAL_COMPILER_RT

[beanz]

Summary

This patch goes with the RFC posted here:
https://discourse.llvm.org/t/rfc-deprecate-and-remove-llvm-build-external-compiler-rt/62058

Compiler-RT is built with way too many different configurations, and we
should reduce duplicateion where possible.

[beanz]

Making the warning message more specific.

[smeenai]

I don't think many people are gonna notice a warning in the general spew of CMake configuration, tbh. Would it be too disruptive to make this an error instead and have a CMake option you can pass to get rid of the error?

[beanz]

Making it an ERROR and providing an option to downgrade it to a WARNING seems reasonable to me. Thoughts?

[delcypher]

Making it an ERROR and providing an option to downgrade it to a WARNING seems reasonable to me. Thoughts?

One nice property this would have is it would (eventually) allow us to collect a list of all the places that depend on this old code. It is potentially quite disruptive though.

[phosek]

I'd be in favor of defaulting to an ERROR and providing an option to downgrade to a WARNING.

[delcypher]

I spoke to a few other engineers in Apple. They are

1. Happy for us to proceed with marking LLVM_BUILD_EXTERNAL_COMPILER_RT as deprecated.
2. Okay with using LLVM_BUILD_EXTERNAL_COMPILER_RT being an error by default that can be downgraded to a warning using a CMake cache option.

@beanz If you can adapt the patch to make the warning into an error with a way to downgrade then I'll happily approve.

[smeenai]

LGTM after changing to an error with a warning downgrade. Resigning to clear my queue, plus the Apple folks should get the final accept here :)

clang/runtime/CMakeLists.txt

 if(NOT EXISTS ${COMPILER_RT_SRC_ROOT})
   # We don't want to set it if LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR is ""
   if(LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR)
     set(COMPILER_RT_SRC_ROOT ${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
   endif()
 endif()
 
 if(LLVM_BUILD_EXTERNAL_COMPILER_RT AND EXISTS ${COMPILER_RT_SRC_ROOT}/)
+  message(WARNING "LLVM_BUILD_EXTERNAL_COMPILER_RT is deprecated and will be "
+                  "removed in LLVM 16, please migrate to "
+                  "LLVM_ENABLE_RUNTIMES=compiler-rt")
 
   # Add compiler-rt as an external project.
   set(COMPILER_RT_PREFIX ${CMAKE_BINARY_DIR}/projects/compiler-rt)
 
   set(STAMP_DIR ${CMAKE_CURRENT_BINARY_DIR}/compiler-rt-stamps/)
   set(BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/compiler-rt-bins/)
 
   add_custom_target(compiler-rt-clear