[flang] Add virtual IntExpr::~IntExpr to silence Clang warning about...

[flang] Add virtual IntExpr::~IntExpr to silence Clang warning about non-virtual dtor in a class with virtual functions.

Original-commit: flang-compiler/f18@b024ba50e1020c05a1bbf63e997e09a2e20d54b8
Reviewed-on: https://github.com/flang-compiler/f18/pull/163
Tree-same-pre-rewrite: false