[DWARF] Fix reading 64-bit DWARF type units.

The type_offset field is 8 bytes long in DWARF64. The patch extends
TypeOffset to uint64_t and fixes its reading. The patch also fixes
checking of TypeOffset bounds as it was inaccurate in DWARF64 case.

Differential Revision: https://reviews.llvm.org/D66465

llvm-svn: 369378