[lldb][NFCI] Redefine dw_attr_t typedef with llvm::dwarf::Attribute

Similar to dw_form_t, dw_attr_t is typedef'd to be a uint16_t. LLVM
defines their type `llvm::dwarf::Attribute` as an enum backed by a
uint16_t. Switching to the LLVM type buys us type checking and the
requirement of explicit casts.

Differential Revision: https://reviews.llvm.org/D150299