[PECOFF] Fix _imp_ implicit symbols.

Implicit symbol for local use implemented in r207141 was not fully
compatible with MSVC link.exe. In r207141, I implemented the feature
in such way that implicit symbols are defined only when they are
exported with /EXPORT option.

After that I found that implicit symbols are defined not only for
dllexported symbols but for all defined symbols. Actually _imp_
implicit symbols have no relationship with the dllexport feature. You
could add _imp_ to any symbol to get a pointer to the symbol, whether
the symbol is dllexported or not.  It looks pretty weird to me but
that's what we want if link.exe behaves that way.

Here is a bit about the implementation: Creating all implicit symbols
beforehand is going to be a huge waste of resource. This feature is
rarely used, and MSVC link.exe even prints out a warning message when
it finds this feature is being used. So we create implicit symbols
on demand. There is an archive file that creates implicit symbols when
they are needed.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207476 91177308-0d34-0410-b5e6-96231b3b80d8