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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287845
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