Don't emit open-paren fixit without close-paren.

getLocForEndOfToken() isn't guaranteed to succeed; if it doesn't, make sure
we do something sane.

Fixes PR16673.  I'm not sure how to write a testcase for this short of grepping
through the diagnostic output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186889 91177308-0d34-0410-b5e6-96231b3b80d8