[flang] Added -fget-definitions and -fget-all-symbols.

- -fget-definitions finds the definition of the symbol under specified source
position.
- -fget-all-symbols finds definition locations of all symbols in a document. For
symbols found in other modules, shows which module the symbol came from.
- Tests.
- New structure SourcePosition with file, line, column information.

Original-commit: flang-compiler/f18@e0099b0900b47ff1d8903c2462bb8a42ecc7999b
Reviewed-on: https://github.com/flang-compiler/f18/pull/698
Tree-same-pre-rewrite: false