Instead of having `State::getLangOpts()`, which does a virtual call to
`getASTContext()` to call `getLangOpts()` on that, just move
`getLangOpts()` to the subclasses so we can do that without the virtual
call. We never call `getLangOpts()` in `State.cpp`, so it's not needed
in the base class.
The new constant interpreter's `clang::interp::InterpState` contains
both `clang::interp::Context` and `clang::ASTContext`. So using `S.Ctx`
and `S.getCtx()` was a bit confusing. This PR rename `getCtx()` to
`getASTContext` to make things more clearer.
Signed-off-by: yronglin <yronglin777@gmail.com>
