Re-commit "[clang][Interp] Unify visiting variable declarations"

We often visit the same variable multiple times, e.g. once when checking
its initializer and later when compiling the function. Unify both of
those in visitVarDecl() and do the returning of the value in
visitDecl().

This time, use a VariableScope instead of a DeclScope for local
variables. This way, we don't emit Destroy ops for the local variables
immediately after creating them.

Differential Revision: https://reviews.llvm.org/D136815