Changes

[Object] Fix the return type of getOffset/getSize
Header64.offset/Header64.size are uint64_t, thus we should not  truncate
them to unit32_t. Moreover, there are a number of places  where we sum
the offset and the size (e.g. in various checks in MachOUniversal.cpp), 
the truncation causes issues since the offset/size can perfectly fit
into uint32_t,  while the sum overflows.
Differential revision: https://reviews.llvm.org/D69126
Test plan: make check-all
llvm-svn: 375154