Make --export-dynamic-symbol to pull out object files from archives.

This is for compatiblity with GNU gold. GNU gold tries to resolve
symbols specified by --export-dynamic-symbol. So, if a symbol specified
by --export-dynamic-symbol is in an archive file, lld's result is
currently different from gold's.

Interestingly, that behavior is different for --dynamic-list.
I added a new test to ensure that.

Differential Revision: https://reviews.llvm.org/D43103

llvm-svn: 324752