[Driver] Fix handling of -fbuiltin/-fcommon when combined with -mkernel

-mkernel enables -fno-builtin and -fno-common by default, but allows -fbuiltin
and -fcommon to override that. However "-fbuiltin -fno-builtin" is treated the
same as "-fbuiltin" which is wrong, so fix that. Also fixes similar behaviour
when -fno-common is default.

Differential Revision: http://reviews.llvm.org/D11459


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244437 91177308-0d34-0410-b5e6-96231b3b80d8