TrailingObjects::FixedSizeStorage constexpr fixes + tests

Summary:
This change fixes issues with `LLVM_CONSTEXPR` functions and
`TrailingObjects::FixedSizeStorage`. In particular, some of the
functions marked `LLVM_CONSTEXPR` used by `FixedSizeStorage` were not
implemented such that they evaluate successfully as part of a constant
expression despite constant arguments.

This change also implements a more traditional template-meta path to
accommodate MSVC, and adds unit tests for `FixedSizeStorage`.

Drive-by fix: the access control for members of `TrailingObjectsImpl` is
tightened.

Reviewers: faisalv, rsmith, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22668

llvm-svn: 277270