llvm-lto: Add testing coverage for local contexts

Add coverage in `llvm-lto` for the API exposed by libLTO to create
modules in local contexts.

The goal here isn't to test the symbol-related API extensively, just to
confirm that these modules work at all.  (I'll be shifting code around
soon that should be NFC and I realized there was no test coverage.)

llvm-svn: 224408