[LLD][COFF] Improve symbol table info for import thunk

Import thunks themselves contain a jump or branch, which is code by
nature. Therefore the import thunk symbol should be marked as function
type in the symbol table to help with debugging.

The `__imp_` import symbol associated to the import thunk is also useful
for debugging. However, when the import symbol isn't directly referenced
outside of the import thunk, it doesn't normally get added to the symbol
table. This change teaches LLD to add the import symbol explicitly.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D134169