Summary:
Commit 512eeb9b
(`More explicit foralls (GHC Proposal 0007)`) introduced breaking
changes to the Template Haskell AST. As a consequence of this, there
are libraries in the wild that now fail to build on GHC HEAD (for
instance, `th-abstraction`).

This properly bumps the `template-haskell` library's version number
to `2.15.0.0` so that these libraries can guard against these changes
using `MIN_VERSION_template_haskell`.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15818

Differential Revision: https://phabricator.haskell.org/D5272