IR: Module summary representation for type identifiers; summary test...

IR: Module summary representation for type identifiers; summary test scaffolding for lowertypetests.

Set up basic YAML I/O support for module summaries, plumb the summary into
the pass and add a few command line flags to test YAML I/O support. Bitcode
support to come separately, as will the code in LowerTypeTests that actually
uses the summary. Also add a couple of tests that pass by virtue of the pass
doing nothing with the summary (which happens to be the correct thing to do
for those tests).

Differential Revision: https://reviews.llvm.org/D28041

llvm-svn: 291069