[cmake] Put check from D110016 behind (default-on) flag

See discussion on https://reviews.llvm.org/D110016 for details.

llvm/CMakeLists.txt

@@ -71,11 +71,15 @@ set(LLVM_EXTRA_PROJECTS "flang")
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
 	"Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
-foreach(proj ${LLVM_ENABLE_PROJECTS})
-  if (NOT proj STREQUAL "all" AND NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
-     MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}")
-  endif()
-endforeach()
+
+option(LLVM_CHECK_ENABLED_PROJECTS "Whether to check LLVM_ENABLE_PROJECTS" ON)
+if (LLVM_CHECK_ENABLED_PROJECTS)
+  foreach(proj ${LLVM_ENABLE_PROJECTS})
+    if (NOT proj STREQUAL "all" AND NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
+       MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}")
+    endif()
+  endforeach()
+endif()
 
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})