Object: Simplify the IRObjectFile symbol iterator implementation.

Change the IRObjectFile symbol iterator to be a pointer into a vector of
PointerUnions representing either IR symbols or asm symbols.

This change is in preparation for a future change for supporting multiple
modules in an IRObjectFile. Although it causes an increase in memory
consumption, we can deal with that issue separately by introducing a bitcode
symbol table.

Differential Revision: https://reviews.llvm.org/D26928

llvm-svn: 287845