[llvm-symbolizer] Make --relative-address work with DWARF contexts

Summary:
Previously the relative address flag only affected PDB debug info.  Now
both DIContext implementations always expect to be passed virtual
addresses. llvm-symbolizer is now responsible for adding ImageBase to
module offsets when --relative-offset is passed.

Reviewers: zturner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12883

llvm-svn: 249784