Add loop pragma for Loop Distribution

Summary:
This is similar to other loop pragmas like 'vectorize'.  Currently it
only has state values: distribute(enable) and distribute(disable).  When
one of these is specified the corresponding loop metadata is generated:

  !{!"llvm.loop.distribute.enable", i1 true/false}

As a result, loop distribution will be attempted on the loop even if
Loop Distribution in not enabled globally.  Analogously, with 'disable'
distribution can be turned off for an individual loop even when the pass
is otherwise enabled.

There are some slight differences compared to the existing loop pragmas.

1. There is no 'assume_safety' variant which makes its handling slightly
different from 'vectorize'/'interleave'.

2. Unlike the existing loop pragmas, it does not have a corresponding
numeric pragma like 'vectorize' -> 'vectorize_width'.  So for the
consistency checks in CheckForIncompatibleAttributes we don't need to
check it against other pragmas.  We just need to check for duplicates of
the same pragma.

Reviewers: rsmith, dexonsmith, aaron.ballman

Subscribers: bob.wilson, cfe-commits, hfinkel

Differential Revision: http://reviews.llvm.org/D19403

llvm-svn: 272656