Don't reduce the stack protector level given -fstack-protector.

Before -fstack-protector would always force a level of 1, even if the default
was 2.

Patch by Brad Smith.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209479 91177308-0d34-0410-b5e6-96231b3b80d8