[CMake] Add missing CheckCSourceCompiles include

[thesamesam]

Summary

Fails with cmake-3.27_rc1 otherwise:

CMake Error at /var/tmp/portage/sys-libs/libcxxabi-17.0.0_pre20230609/work/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)

Needed for CHECK_C_SOURCE_COMPILES.

Bug: https://bugs.gentoo.org/908233

[mgorny]

LGTM. Thanks!

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(CheckCXXCompilerFlag)
 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()