[lld-macho] Initial support for common symbols

On Unix, it is traditionally allowed to write variable definitions without
initialization expressions (such as "int foo;") to header files. These are
called tentative definitions.

The compiler creates common symbols when it sees tentative definitions. When
linking the final binary, if there are remaining common symbols after name
resolution is complete, the linker converts them to regular defined symbols in
a `__common` section.

This diff implements most of that functionality, though we do not yet handle
the case where there are both common and non-common definitions of the same
symbol.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D86909