[CMake] Fixes using CMake 3.27.0.

[Mordante]

Summary

Testing libc++ with CMake 3.27.0-rc1 fails. It seems some of the modules
used in CMake are not included.

The error before this change was

CMake Error at <mono-repo>/llvm/cmake/modules/HandleLLVMOptions.cmake:821 (CHECK_C_SOURCE_COMPILES):
  Unknown CMake command "CHECK_C_SOURCE_COMPILES".
Call Stack (most recent call first):
  CMakeLists.txt:156 (include)

[kwk]

Can you please how things fail? What is the error message for example?

[Mordante]

In D152619#4413263, @kwk wrote:

Can you please how things fail? What is the error message for example?

Good point! I updated the commit message.

[kwk]

In D152619#4413601, @Mordante wrote:

In D152619#4413263, @kwk wrote:

Can you please how things fail? What is the error message for example?

Good point! I updated the commit message.

Thank you.

Can I assume that the follow-up error after adding include(CheckCSourceCompiles) is similar but with CHECK_CXX_SOURCE_COMPILES?

Could this be an error in CMake itself that should be fixed upstream?

[Mordante]

In D152619#4413892, @kwk wrote:

In D152619#4413601, @Mordante wrote:

In D152619#4413263, @kwk wrote:

Can you please how things fail? What is the error message for example?

Good point! I updated the commit message.

Thank you.

Can I assume that the follow-up error after adding include(CheckCSourceCompiles) is similar but with CHECK_CXX_SOURCE_COMPILES?

Could this be an error in CMake itself that should be fixed upstream?

Based on the documentation https://cmake.org/cmake/help/latest/module/CheckCSourceCompiles.html this module provides CHECK_C_SOURCE_COMPILES. So I think we forgot an include and it just happened to work.

I expect this https://github.com/Kitware/CMake/commit/03c6ebf2b53956d9d8d035f9bf67bb73caca65f3 caused the issue
CheckCCompilerFlag.cmake removed its CheckCSourceCompiles include.

[thesamesam]

Yes, it's a transitive include which just happened to work before.

See https://reviews.llvm.org/D152622 for CheckCSourceCompiles. I'd missed this change set.

llvm/cmake/modules/HandleLLVMOptions.cmake

 # This CMake module is responsible for interpreting the user defined LLVM_
 # options and executing the appropriate CMake commands to realize the users'
 # selections.
 
 # This is commonly needed so make sure it's defined before we include anything
 # else.
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 include(CheckCompilerVersion)
 include(CheckProblematicConfigurations)
 include(HandleLLVMStdlib)
 include(CheckCCompilerFlag)
+include(CheckCSourceCompiles)
 include(CheckCXXCompilerFlag)
+include(CheckCXXSourceCompiles)
 include(CheckSymbolExists)
-include(CheckCSourceCompiles)
 include(CMakeDependentOption)
 include(LLVMProcessSources)
 
 if(CMAKE_LINKER MATCHES ".*lld" OR (LLVM_USE_LINKER STREQUAL "lld" OR LLVM_ENABLE_LLD))
   set(LINKER_IS_LLD TRUE)
 else()
   set(LINKER_IS_LLD FALSE)
 endif()