Workaround for EvalInfo ctor for MSVC 2017

Current EvalInfo ctor causes EnableNewConstInterp to be true even though
it is supposed to be false on MSVC 2017. This is because a virtual function
getLangOpts() is called in member initializer lists, whereas on MSVC
member ctors are called before function virtual function pointers are
initialized.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D70729