[GHC] #15818: Bump template-haskell version to 2.15.0.0

[GHC]

#15818: Bump template-haskell version to 2.15.0.0
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Template       |           Version:  8.7
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #2600, #14268
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Commit 512eeb9bb9a81e915bfab25ca16bc87c62252064 (`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, [https://github.com/glguy/th-
 abstraction/issues/55 th-abstraction]). We should properly bump 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`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15818>
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