[Makefile] If ENABLE_CLANG_ARCMT=0 it seems we don't build libclang, so the unittests for libclang should be disabled too.

This is a modified patch provided by Vassil Vassilev.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220965 91177308-0d34-0410-b5e6-96231b3b80d8
[libclang] Disable spell-checking and warnings during code-completion since they are not needed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220966 91177308-0d34-0410-b5e6-96231b3b80d8