Continuing from D77388, this patch moves interface declarations associated with MachODump.cpp into the headers corresponding to the file that defines the variable. At the same time, these externs are moved into the llvm::objdump namespace. The externs defined in MachODump.cpp that are not referenced outside of it are given internal linkage.
This patch does not rename the external functions defined by MachODump.cpp that are not clearly named as being specific to Mach-O.