IR: Canonicalize metadata formatting, NFC

Canonicalize formatting of metadata to make it easier to upgrade via
scripts -- in particular, one line per metadata definition makes it more
`sed`-able.

This is preparation for changing the assembly syntax for metadata [1].

[1]: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20141208/248449.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224002 91177308-0d34-0410-b5e6-96231b3b80d8