[codeview] use 32-bit integer for RelocOffset in DebugLinesSubsection

Summary:
RelocOffset is a 32-bit value, but we previously truncated it to 16 bits.

Fixes PR33335.

Reviewers: zturner, hiraditya!

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33968

llvm-svn: 305043