[DebugInfo] Make AsmPrinter struct HandlerInfo and Handlers protected

In AsmPrinter, make struct HandlerInfo and SmallVector
Handlers protected, so target extended AsmPrinter will
be able to add their own handlers.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D55756

llvm-svn: 349602