[LLD][COFF] Reduce chance of symbol name collision with delay-load

Delay-loaded imports creats a load thunk with a symbol name. Before this
change, the name uses a `__imp_load_` prefix. On the other hand, normal
import uses the `__imp_` prefix for the import address pointer. If an
import symbol named `load_func` is imported normally and another named
`func` is imported using delay-load, this can cause a symbol name
collision.

This patch changes delay-load imports to use `__imp___load_` prefix.
Because it is less likely for normal imports to have a name starting in
`__load_` this should reduce the chance of a name collision.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D134464