Fixup debug information for the location information for __block

Fixup debug information for the location information for __block
variables.  For this to work, the backend needs to handle more complex
forms for locations.

A typical utterance would be:

        %forwarding = getelementptr %0* %use_by_ref, i32 0, i32 1               ; <i8**> [#uses=1]
        %0 = load i8** %forwarding              ; <i8*> [#uses=1]
        %1 = bitcast i8* %0 to %0*              ; <%0*> [#uses=1]
        %x = getelementptr %0* %1, i32 0, i32 4         ; <i32*> [#uses=1]
        %2 = bitcast i32* %x to { }*            ; <{ }*> [#uses=1]
        call void @llvm.dbg.declare({ }* %2, { }* bitcast (%llvm.dbg.variable.type* @llvm.dbg.variable to { }*))

Presently when selection finds something it doesn't understand, it
just avoids generating any information, which is safe, just
incomplete.  Radar 6867696

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71824 91177308-0d34-0410-b5e6-96231b3b80d8