[ThinLTO] Add option to dump per-module summary dot graph

Summary:
I found that there currently isn't a way to invoke exportToDot from
the command line for a per-module summary index, and therefore no
testing of that case. Add an internal option and use it to test dumping
of per module summary indexes.

In particular, I am looking at fixing the limitation that causes the
aliasee GUID in the per-module summary to be 0, and want to be able to
test that change.

Reviewers: evgeny777

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D57206

llvm-svn: 352441