Do not allow delay-importing data symbols.

If you pass /delayload:<dllname> to the COFF linker, it creates thunks
so that DLLs are loaded when they are used for the first time instead of
load-time.

This mechanism do not work for data symbols as there's no way to trap
acccesses to data imported from DLLs. (Technically, I think if we do not
initially map dllimport tables in memory, we could actually trap accesses
and delay-load data symbols, but that's not what Windows do.)

This patch is to report an error when you try to delay-load data symbols.

Fixes https://bugs.llvm.org/show_bug.cgi?id=33106

Differential Revision: https://reviews.llvm.org/D33557

llvm-svn: 303890