[clang] Fix an uint32_t overflow in large preamble.

Summary:
I was surprised to see the LocalOffset can exceed uint32_t, but it
does happen and lead to crashes in one of our internal huge TU with a large
preamble.

with this patch, the crash is gone.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79397