[dsymutil] Fix __LINKEDIT vmsize in dsymutil upgrade path

Summary:
dsymutil upgrade path can change the size of segment and it needs to update
the vmsize of the segment to reflect the size change.

rdar://problem/30290714

Reviewers: friss

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29433

llvm-svn: 293842