[Dwarf] Make dw_tag_t a typedef for llvm::dwarf::Tag instead of uint16_t.

Currently dw_tag_t is a typedef for uint16_t. This patch changes makes
dw_tag_t a typedef for llvm::dwarf::Tag. This enables us to use the full
power of the DWARF utilities in LLVM without having to do the cast every
time. With this approach, we only have to do the cast when reading the
ULEB value.

Differential revision: https://reviews.llvm.org/D68005

llvm-svn: 372891