[clang][Interp] Unify visiting variable declarations

[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().

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