Re-insert flang-rt dependency check

Author: Meinersbur

llvm/CMakeLists.txt

@@ -144,6 +144,12 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   endif()
 endif()
 
+if ("flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES)
+  if (NOT "flang" IN_LIST LLVM_ENABLE_PROJECTS)
+    message(FATAL_ERROR "Flang is not enabled, but is required for the Flang-RT runtime")
+  endif ()
+endif ()
+
 # Select the runtimes to build
 #
 # As we migrate runtimes to using the bootstrapping build, the set of default runtimes