Revert "coff also doesn't have a ReadOnlySection yet, (!)"

This reverts commit r77814.

We were sticking global constants in the .data section instead of in the
.rdata section when emitting for COFF.

This fixes PR16831.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187956 91177308-0d34-0410-b5e6-96231b3b80d8