Reland [DWARF] Add a unit test for DWARFUnit::getLength().

This is a follow-up of rL369529, where the return value of
DWARFUnit::getLength() was changed from uint32_t to uint64_t.
The test checks that a unit header with Length > 4G can be successfully
parsed and the value of the Length field is not truncated.

Differential Revision: https://reviews.llvm.org/D67276

llvm-svn: 371510