[DwarfDebug] Move MergeValues to .cpp, NFC
Pull in r257979 from upstream llvm trunk, by Keno Fischer:
[DwarfDebug] Don't merge DebugLocEntries if their pieces overlap
Summary:
Later in DWARF emission we check that DebugLocEntries have
non-overlapping pieces, so we should create any such entries
by merging here.
Fixes PR26163.
Reviewers: aprantl
Differential Revision: http://reviews.llvm.org/D16249
Again, these will be merged to the official release_38 branch soon, but
we need them ASAP.