MathExtras.h: add LLVM_CONSTEXPR where simple

Summary:
This change adds `LLVM_CONSTEXPR` to functions selected as follows:
- the body is already valid under C++11 for a `constexpr` function,
- the evaluation of the function, given constant arguments, will not
  fail during the evaluation of a constant expression, and
- the above properties are easily verifiable at a glance.

Note: the evaluation of the function cannot fail if the instantiation
triggers a static assertion failure.

Reviewers: faisalv, rsmith, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22824

llvm-svn: 277269