[lldb][COFF] Add note to forwarder export symbols in symtab

Forwarder exports do not point to a real function or variable. Instead
they point to a string describing which DLL and symbol to forward to.
Any imports which uses them will be redirected by the loader
transparently. These symbols do not have much use in LLDB, but keep them
just in case someone find it useful. Also set a synthesized name with
the forwarder string for informational purpose.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D134518