Define LLVM_ENABLE_WARNINGS when building standalone clang to ensure warnings get enabled

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297106 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -82,6 +82,7 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   endif()
 
+  option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
   option(LLVM_INSTALL_TOOLCHAIN_ONLY
     "Only include toolchain files in the 'install' target." OFF)