[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