COFF: Merge .idata, .didat and .edata into .rdata by default.

This saves a little space and matches what link.exe does.

Tested using the chromium Windows trybots:
https://chromium-review.googlesource.com/c/chromium/src/+/1014784

Differential Revision: https://reviews.llvm.org/D45737

llvm-svn: 330223