IR: Function summary representation for type tests.

Each function summary has an attached list of type identifier GUIDs. The
idea is that during the regular LTO phase we would match these GUIDs to type
identifiers defined by the regular LTO module and store the resolutions in
a top-level "type identifier summary" (which will be implemented separately).

Differential Revision: https://reviews.llvm.org/D27967

llvm-svn: 290280