Make 32-bit member offset in Archive::Symbol::getMember 64-bit

When accessing a member for a symbol with an offset greater than 2^32 -
1 the current Archive::Symbol::getMember implementation will overflow
and cause unexpected behavior. This change simply fixes that. In
particular if you call "llvm-nm --print-armap" on an archive that has
this behavior you'll get an error.

Differential Revision: https://reviews.llvm.org/D39379

llvm-svn: 316801