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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277270
91177308-0d34-0410-b5e6-
96231b3b80d8