[llvm][cmake] Add clang if not already present when building lldb (#149055)

Fixes #54555

This follows flang's pattern, it adds clang if you don't have it in
LLVM_ENABLE_PROJECTS.

Author: DavidSpickett

llvm/CMakeLists.txt

@@ -179,6 +179,13 @@ 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(STATUS "Enabling clang as a dependency of lldb")
+    list(APPEND LLVM_ENABLE_PROJECTS "clang")
+  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 "