[DebugInfo] Fix truncation of call site immediates

Summary:
This addresses a bug in collectCallSiteParameters() where call site
immediates would be truncated from int64_t to unsigned.

This fixes PR43525.

Reviewers: djtodoro, NikolaPrica, aprantl, vsk

Reviewed By: aprantl

Subscribers: hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D68869

llvm-svn: 374770