[Inliner] Report when inlining fails because callee's def is unavailable

Summary:
This is obviously an interesting case because it may motivate code
restructuring or LTO.

Reporting this requires instantiation of ORE in the loop where the call
sites are first gathered.  I've checked compile-time
overhead *with* -Rpass-with-hotness and the worst slow-down was 6% in
mcf and quickly tailing off.  As before without -Rpass-with-hotness
there is no overhead.

Because this could be a pretty noisy diagnostics, it is currently
qualified as 'verbose'.  As of this patch, 'verbose' diagnostics are
only emitted with -Rpass-with-hotness, i.e. when the output is expected
to be filtered.

Reviewers: eraman, chandlerc, davidxl, hfinkel

Subscribers: tejohnson, Prazek, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D23415

llvm-svn: 279860