[Flang] Implicitly add clang if flang enabled (llvm#123964)

Clang is required to compile Flang. Instead of erroring-out if Clang is
enabled, for convinience implicitly add it to `LLVM_ENABLE_PROJECTS`,
consistent with how the MLIR dependency is handled.

This is motivatated by the discussion on whether flang-rt shoud be enabled implicitly 
(https://discourse.llvm.org/t/buildbot-changes-with-llvm-enable-runtimes-flang-rt/83571/2).
Since the answer was yet, clang would have been the only exception of not being
enabled implicitly. Fixed with this commit.

Author: Meinersbur

llvm/CMakeLists.txt

@@ -146,7 +146,8 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   endif()
 
   if (NOT "clang" IN_LIST LLVM_ENABLE_PROJECTS)
-    message(FATAL_ERROR "Clang is not enabled, but is required for the Flang driver")
+    message(STATUS "Enabling clang as a dependency to flang")
+    list(APPEND LLVM_ENABLE_PROJECTS "clang")
   endif()
 endif()