[ConstExprPreter] Updated constant interpreter documentation

Summary:
Updated the documentation to better reflect features implemented on the
constexpr branch at https://github.com/nandor/llvm-project and extended
the TODO list with known missing features

Reviewers: rsmith, Bigcheese, dexonsmith, jfb

Subscribers: lebedev.ri, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75726