[flang] Fix issues comming from clang-10 warnings

- Remove SubprogramDetails copy ctor
- Prevent copies in range based loops over symbols
- Remove unsued var

Original-commit: flang-compiler/f18@16543d22f74e9421ecb4078818f4c1970bac0a5d
Reviewed-on: https://github.com/flang-compiler/f18/pull/972