DebugInfo: Check for null before using DIType
DebugInfo: Check for null before using DIType

A WIP patch to turn on stricter `DIDescriptor` accessor checks fires
here; it's obvious from the code that `T` can be null, so add an
explicit check.  Caught by dozens of current testcases.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@232791 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang