[COFF] Allow exporting all symbols from system libraries specfied with -wholearchive:

When building a shared libc++.dll, it pulls in libc++abi.a statically
with the --wholearchive flag. If such a build is done with
--export-all-symbols, it's reasonable to assume that everything
from that library also should be exported with the same rules as normal
local object files, even though we normally avoid autoexporting things
from libc++abi.a in other cases when linking a DLL (user code).

Differential Revision: https://reviews.llvm.org/D51529

llvm-svn: 341403