[CMake] Add missing CheckCSourceCompiles include

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

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D152622