Avoid PointerIntPair of constexpr EvalInfo structs

They are stack allocated, so their alignment is not to be trusted.
32-bit MSVC only guarantees 4 byte stack alignment, even though alignof
would tell you otherwise. I tried fixing this with __declspec align, but
that apparently upsets GCC. Hopefully this version will satisfy all
compilers.

See PR32018 for some info about the mingw issues.

Should supercede https://reviews.llvm.org/D34873

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@310905 91177308-0d34-0410-b5e6-96231b3b80d8