[dsymutil] Fix handling of common symbols in multiple object files.

For common symbols the linker emits only a single symbol entry in the
debug map. This caused dsymutil to not relocate common symbols when
linking DWARF coming form object files that did not have this entry.
This patch fixes that by keeping track of common symbols in the object
files and synthesizing a debug map entry for them using the address from
the main binary.

Differential revision: https://reviews.llvm.org/D68680

llvm-svn: 374139