Fix getting section info in large mach-o files. (#165940)
Mach-o has 32 bit file offsets in the MachO::section_64 structs. dSYM
files can contain sections whose start offset exceeds UINT32_MAX, which
means the MachO::section_64.offset will get truncated. We can calculate
when this happens and properly adjust the section offset to be 64 bit
safe. This means tools can get the correct section contents for large
dSYM files and allows tools that parse DWARF, like llvm-gsymutil, to be
able to load and convert these files correctly.
