Improve debug info generated with enabled AddressSanitizer.

When ASan replaces <alloca instruction> with
<offset into a common large alloca>, it should also patch
llvm.dbg.declare calls and replace debug info descriptors to mark
that we've replaced alloca with a value that stores an address
of the user variable, not the user variable itself.

See PR11818 for more context.

llvm-svn: 169984