Fix a -Wparentheses warning from GCC that caught a badly formed assert.

I have no idea why Clang's warning doesn't fire here, looks like a Clang
bug. I'll investigate that separately.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198677 91177308-0d34-0410-b5e6-96231b3b80d8