Add FixedSizeStorage to TrailingObjects; NFC

Summary: This change introduces two types, `FixedSizeStorage` and `FixedSizeStorageOwner`, which can be used to provide stack-allocated objects with trailing objects.

Reviewers: rsmith, faisalv, aaron.ballman

Subscribers: llvm-commits, cfe-commits, nwilson

Differential Revision: http://reviews.llvm.org/D19770

llvm-svn: 273664