[PECOFF] Emit the import table to .idata section.

Emitting idata atoms to their own section would make debugging easier.
The Windows loader do not really care about whether the DLL import table is
in .rdata or its own .idata section, so there is no change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196458 91177308-0d34-0410-b5e6-96231b3b80d8