COFF: Continue reading object files until converge.

In this linker model, adding an undefined symbol may trigger chain
reactions. It may trigger a Lazy symbol to read a new file.
A new file may contain a directive section, which may contain various
command line options.

Previously, we didn't handle chain reactions well. We visited /include'd
symbols only once, so newly-added /include symbols were ignored.
This patch fixes that bug.

Now, the symbol table is versioned; every time the symbol table is
updated, the version number is incremented. We repeat adding undefined
symbols until the version number does not change. It is guaranteed to
converge -- the number of undefined symbol in the system is finite,
and adding the same undefined symbol more than once is basically no-op.

llvm-svn: 240177