[llvm][cmake] Error if lldb project is enabled without clang

Fixes #54555

We could do what flang does and enable clang automatically,
but I personally prefer making the user make the choice.

Also from a buld requirements perspective, flang is a bigger
build than clang. So if you're already set on building flang,
clang should be within your budget too. lldb is smaller than
clang.

Author: DavidSpickett

llvm/CMakeLists.txt

@@ -179,6 +179,12 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   endif ()
 endif()
 
+if ("lldb" IN_LIST LLVM_ENABLE_PROJECTS)
+  if (NOT "clang" IN_LIST LLVM_ENABLE_PROJECTS)
+    message(FATAL_ERROR "Clang is not enabled, but is required for lldb.")
+  endif ()
+endif ()
+
 if ("libc" IN_LIST LLVM_ENABLE_PROJECTS)
   message(WARNING "Using LLVM_ENABLE_PROJECTS=libc is deprecated.  Please use "
     "-DLLVM_ENABLE_RUNTIMES=libc or see the instructions at "