Remove problematic PrettyStackTrace entry added in r328276
Remove problematic PrettyStackTrace entry added in r328276

I'm not sure /why/ this is causing issues for libclang, but it is.
Unbreak the buildbots since it's already consumed an hour of my time.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@328286 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang