Add /bigobj for SPIRV dialect on MSVC.

Summary: This object file has grown beyond the default limit, and elsewhere in LLVM, we seem to be setting this flag as a one-off, so continuing that here.

Reviewers: mravishankar, antiagainst

Subscribers: mgorny, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, bader, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77002