[llvm][cmake] Performing expensive checks requires enabling assert. (llvm#80821)

LLVM will intercept errors using assert() when
LLVM_ENABLE_EXPENSIVE_CHECKS is ON. So an explicit check is added.

---------

Co-authored-by: Hua Tian <[email protected]>

Author: huaatian

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -128,6 +128,11 @@ if("${LLVM_DEFAULT_TARGET_TRIPLE}" MATCHES "^amdgcn" OR
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)
+  # When LLVM_ENABLE_EXPENSIVE_CHECKS is ON, LLVM will intercept errors
+  # using assert(). An explicit check is performed here.
+  if (NOT LLVM_ENABLE_ASSERTIONS)
+    message(FATAL_ERROR "LLVM_ENABLE_EXPENSIVE_CHECKS requires LLVM_ENABLE_ASSERTIONS \"ON\".")
+  endif()
   add_compile_definitions(EXPENSIVE_CHECKS)
 
   # In some libstdc++ versions, std::min_element is not constexpr when