Tentatively enable -Wcast-calling-convention by default

In Chrome, this would have found two true positives around CreateThread
if we hadn't already fixed them while rolling out ASan. We didn't get
any other hits in Chrome. I'm curious to hear if this warning finds
anything in other projects.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269780 91177308-0d34-0410-b5e6-96231b3b80d8