This flag reports a summary of the inlining decision for identifiers
prefixed by the flag's argument.

For example, `-dinline-check foo` will report why definitions whose
prefix is `foo` are inlined or not.

Previously the only way to get this information was to pass a
combination of `-dverbose-core2core` and `-ddump-inlinings`.

This combination led to a log of 12 million lines in a module of about
200 lines I recently had to apply it to. This flag provides a much more
direct way to find the occurence you care about.

Reviewers: osa1, dfeuer, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4458