[DebugInfo] Pass all values in DebugLocEntry's constructor, NFC

Summary:
With MergeValues() removed, amend DebugLocEntry's constructor so that it
takes multiple values rather than a single, and keep non-fragment values
in OpenRanges, as this allows some cleanup of the code in
buildLocationList().

Reviewers: aprantl, dblaikie, loladiro

Reviewed By: aprantl

Subscribers: hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D59303

llvm-svn: 357988