[COFF] Assign unique names to autogenerated .weak.<name>.default symbols

These symbols need to be external (MSVC tools error out if a weak
external points at a symbol that isn't external; this was tried before
but had to be reverted in bc5b7217,
and this was originally explicitly fixed in
732eeaf2).

If multiple object files have weak symbols with defaults, their
defaults could cause linker errors due to duplicate definitions,
unless the names of the defaults are unique.

GNU binutils handles this by appending the name of another symbol
from the same object file to the name of the default symbol. Try
to implement something similar; before writing the object file,
locate a symbol that should have a unique name and use the name of
that one for making the weak defaults unique.

Differential Revision: https://reviews.llvm.org/D75989