add -th-file which generates a th.hs file

Summary:
see Trac #8624

similar functionality is now available
with -ddump-to-file -ddump-splices

However, users are already accustomed to -ddump-splices
having a particular format, and this format is not completely valid code
The goal of -th-file is to dump valid Haskell code

Additionally, the convention of -ddump-to-file is to name the file after
the flag, so the file is .dump-splices
Given that the goal of the new flag is to generate valid Haskell,
The extension should be .hs

Additionally, -ddump-to-file effects all other dump flags

Test Plan:
look at the output of using the -th-file flag
and compare it to the output of using -ddump-to-file and -ddump-splices
I want to add test cases, but just need some pointers on getting started there

Reviewers: thomie, goldfire, simonpj, austin

Reviewed By: simonpj, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D518

GHC Trac Issues: #8624

(cherry picked from commit 07ace5c2)

Conflicts:
	docs/users_guide/7.12.1-notes.xml